availability of solar radiation

Assignment 2Objective: Study availability of solar radiation at three locations in Norway and discuss on
use of solar water heater having a capacity of 1000 litres/day.
Approach: Use PVGIS online (web) tool for selected location.
https://re.jrc.ec.europa.eu/pvg_tools/en/
Methodology:
Obtain typical meteorological year (TMY) data for selected locations for a time frame of 10
years (approximately).
A) Plot global horizontal irradiance (monthly basis- along X-axis), Daily irradiance
(hourly basis- along X-axis) for the months of March, June, September and
December.
B) Plot monthly solar radiation (months along x-axis) for last three years (2018, 2019,
2020) for chosen tilt angle of collector.
Parameters to use:
Use Date base: PVGIS-SARAH.
Use the collector slope of 0°, 30°, 45°, 60°, 90° (any value of tilt angle, you can select) for a solar
collector for part B)
Results and Discussion:
Get the above plots.
Discussion on availability of solar radiation as months wise, change in inclination.
Illustrated Examples of results:
Ref:
Anette Tangård, Analysis of solar thermal systems for domestic hot water production in a
nursing home. Master’s Thesis 2019
Master’s Thesis 2019
30 ECTS
Faculty of Science and Technology
Analysis of solar thermal systems
for domestic hot water production
in a nursing home
Anette Tangård
Environmental Physics and Renewable Energy
PREFACE
This thesis is the finishing piece of my master’s degree in Environmental Physics and
Renewable Energy at the Norwegian University of Life Sciences (NMBU). It reflects 4.5
months of work, from the preliminary ideas of its content to the completion of this paper. During
these months, I have, above all, learned that time is precious. I have also learned to appreciate
the value of a second, or more, pair of eyes to evaluate one’s work.
From the time I first heard of the project VarmtVann2030, I found the issues interesting and
meaningful. This was the main reason for choosing to collaborate with SINTEF Byggforsk,
ending in this thesis, “Analysis of solar thermal systems for domestic hot water production in a
nursing home”. The specifications of the thesis were determined in close cooperation with Åse
Lekang Sørensen and Harald Taxt Walnum in SINTEF Byggforsk. I would like to thank these
two for excellent guidance and support during this process.
I would also like to thank my supervisor at NMBU, Jorge Mario Marchetti, for valuable
feedback when needed. Additionally, I am grateful to Malin Helander in SGP Armatec for her
unbound cooperation and Matthias Haase in SINTEF Byggforsk for his introduction into
Polysun. To my family and friends, thank you for always being there.
I hope you enjoy your reading.
Anette Tangård
Tønsberg, 13.05.2019
i
ii
ABSTRACT
The current state of climate change urges the world to consider alternatives concerning the use
of energy. In Norway, electricity is a common energy source in buildings, but heating is a
purpose which can be conducted using other energy carriers. In 2017, SINTEF Byggforsk and
NTNU initiated the project VarmtVann2030 to improve the knowledge about the use of
domestic hot water (DHW) in the country. This thesis examines the possibilities of using solar
collectors as energy source for the heating of DHW in a nursing home. Some of the results are
based on measurements carried out on a nursing home in Drammen, as a part of
VarmtVann2030.
The capacity of the sun is 15 000 times larger than the earth’s population’s total need for energy.
Solar collectors transform radiation energy from the sun into heat, which again is transferred to
an energy carrier, most often a liquid. A solar thermal facility is usually dimensioned to produce
300-600 kWh/m2 sc and cover 40-60 % of the energy needed for DHW during a year. The annual
DHW energy demand for the Drammen nursing home is 53.9 MWh. The existing standard on
DHW energy use at nursing homes, SN/TS 3031, gives consumption values which are almost
twice as large.
Simulations were done using a software called Polysun Designer and calculations were
performed in Excel. The focus was on a pressurised system in combination with an electric
water heater. A solar thermal system was chosen based on advices from SGP Armatec, a
supplier of pressurised installations in Norway. SGP Armatec also offered examples of prices
of materials. Considering different sizes of solar collector areas and accumulator tanks, the most
profitable solution was found. The most profitable system was the one with the lowest Levelised
Cost of Energy (LCOE) out of solar collector areas of 10-100 m2 with accumulator tank
dimensions of 50 l/m2sc, 62.5 l/m2sc and 75 l/m2 sc. The best tilt angle was found doing specified
simulations. In addition to the LCOE, the payback period and annual cost were considered.
Technical parameters included in the results were the solar fraction, area specific collector field
yield and maximum collector temperature.
The most profitable system based on the collected consumption data from the nursing home
consisted of a solar collector area of 40 m2 with a tilt angle of 50° and an accumulator tank of
2000 l. For this solution, the LCOE was 66.9 øre/kWh, the payback period was 23.2 years and
iii
the annual cost was 17 798 NOK/year. The solar fraction was 38 %, the area specific field yield
was 512 kWh/m2 sc and the maximum collector temperature was 90 °C.
Alterations in accumulator tank volume and collector area gave various effects in the
parameters. A large tank gave the best technical performance because of the increased storage
capacity and the lowest economic values occurred for a tank of 1500 l. Regarding construction
size, a small system achieved better outcomes than a large one due to its adaptation to the DHW
consumption, but the one at 40 m2 was most profitable. For the large system (80 m2), the LCOE
was 75.7 øre/kWh, the payback period was 27.2 years, the solar fraction was 56 %, the area
specific field yield was 388 kWh/m2 sc and the maximum collector temperature was 130 °C. For
the small system (20 m2), the LCOE was 73.9 øre/kWh, the payback period was 26.3 years, the
solar fraction was 22 %, the area specific field yield was 594 kWh/m2 sc and the maximum
collector temperature was 76 °C. The annual cost was subject to negligible changes for different
system sizes.
Sensitivity analyses were done on the most profitable system for both the investment cost and
the electricity price, with alterations of ± 30 %. Not surprisingly, all the economic parameters
favoured a low investment cost. The minimum values were an LCOE of 46.8 øre/kWh, a
payback period of 15.2 years and an annual cost of 15 928 NOK/year. For variations in the
electricity price, changes in the LCOE was negligible. The payback period and annual cost was
subject to larger effects, their lowest values being 16.9 years and 14 329 NOK/year,
respectively.
Additional outcomes of the thesis research gave indications that the DHW consumption should
be of a certain magnitude for the use of solar collectors to be adequately profitable. A tripling
of the Drammen nursing home DHW demand gave an LCOE of 53.3 øre/kWh. Simulations of
a demand based on SN/TS 3031 gave reason to believe that the standard overestimates the best
size of solar thermal facilities for nursing homes. SN/TS 3031 resulted in a most profitable
system size of 50 m2. All the parameters, with an exception of the annual cost and solar fraction,
achieved worse results than expected from the standard when implementing the measured
DHW consumption on the 50-m2 construction. This kind of estimation of the demand can give
very different outcomes than predicted.
iv
The results in this thesis show the importance of enhanced research on the use of domestic hot
water. Both costs and use of energy can be minimised if the actual consumption of the building
in each individual case is examined in advance of the installation of a solar thermal construction.
A decrease in the costs of solar thermal facilities and/or an increase in the electricity price would
make it a more desirable alternative.
v
vi
SAMMENDRAG
De pågående klimaendringene stiller krav om omstillinger innen verdens energibruk. I Norge
er elektrisitet er en vanlig energikilde i bygg, men oppvarming kan utføres ved hjelp av andre
energibærere. I 2017 satte SINTEF Byggforsk og NTNU i gang prosjektet VarmtVann2030,
som har som mål å øke kunnskapen innen bruk av varmtvann her i landet. Denne
masteroppgaven undersøker mulighetene for bruk av solfangere som energikilde til å varme
opp varmtvann i et sykehjem. Noen av resultatene er basert på målinger utført ved et sykehjem
i Drammen, som en del av VarmtVann2030.
Solens kapasitet er 15 000 ganger større enn hele jordens befolknings energibehov. Solfangere
omformer strålingsenergien fra sola til varme, som igjen overføres til en energibærer, som oftest
består av en væske. Et solfangeranlegg dimensjoneres normalt for å produsere 300-600
kWh/m2 sc og dekke 40-60 % av varmtvannsenergibehovet i løpet av et år. Den årlige
energibruken til varmtvann på sykehjemmet i Drammen er på 53.9 MWh. Den eksisterende
standarden for energibruk til varmtvann på sykehjem, SN/TS 3031, gir forbruksverdier som er
nesten dobbelt så høye.
Simuleringene ble gjort ved hjelp av en programvare kalt Polysun Designer og utregningene
ble utført i Excel. Fokuset var på et trykksatt system i kombinasjon med en elbereder. Valg av
solfangersystem ble basert på råd fra SGP Armatec AS, en leverandør av trykksatte
installasjoner i Norge. SGP Armatec tilbød også eksempler på materialpriser. En vurdering av
ulike størrelser av solfangerarealer og akkumulatortanker ledet til den mest lønnsomme
løsningen. Det mest lønnsomme systemet var det med den laveste energikostnaden over
levetiden (LCOE) av solfangerarealer på 10-100 m2, og akkumulatortankdimensjoner
tilsvarende 50 l/m2sc, 62.5 l/m2sc og 75 l/m2 sc. Den beste helningsvinkelen ble funnet ved gitte
tester. I tillegg til LCOE ble tilbakebetalingstiden og den årlige kostnaden vurdert. Tekniske
parametere inkludert i resultatene var solfraksjonen, energiutbytte per solfangerareal og
maksimal kollektortemperatur.
Det mest lønnsomme systemet basert på oppsamlede forbruksdata fra sykehjemmet bestod av
et solfangerareal på 40 m2 med en helningsvinkel på 50° og en akkumulatortank på 2000 l. For
denne løsningen ble LCOE 66.9 øre/kWh, tilbakebetalingstiden 23.2 år og den årlige kostnaden
vii
17 798 NOK/år. Solfraksjonen var på 38 %, energiutbyttet per solfangerareal var 512 kWh/m2 sc
og maksimal kollektortemperatur var 90 °C.
Forandringer i akkumulatortankvolum og solfangerareal gav endringer i de forskjellige
parameterne. En stor tank var best ut fra et teknisk ståsted på grunn av den økte
lagringskapasiteten og en 1500-l tank gav de laveste økonomiske verdiene. I forbindelse med
de ulike systemstørrelsene oppnådde et lite system bedre resultater enn et stort fordi det var
bedre tilpasset varmtvannsbehovet, men systemet på 40 m2sc var mest lønnsomt. For det store
systemet (80 m2) var LCOE 75.7 øre/kWh, tilbakebetalingstiden 27.2 år, solfraksjonen 56 %,
energiutbyttet per solfangerareal 388 kWh/m2 sc og maksimal kollektortemperatur 130 °C. For
det lille systemet (20 m2) var LCOE 73.9 øre/kWh, tilbakebetalingstiden 26.3 år, solfraksjonen
22 %, energiutbyttet per solfangerareal 594 kWh/m2 sc og maksimal kollektortemperatur 76 °C.
Den årlige kostnaden endret seg svært lite for ulike systemstørrelser.
Sensitivitetsanalyser på det mest lønnsomme systemet ble utført for både investeringskostnaden
og elektrisitetsprisen, med endringer tilsvarende ± 30 %. Som forventet gav en lav
investeringskostnad i de beste økonomiske resultatene. Minimumsverdiene var en LCOE på
46.8 øre/kWh, en tilbakebetalingstid på 15.2 år og en årlig kostnad på 15 928 NOK/år. LCOE
endret seg svært lite for variasjoner i elektrisitetsprisen. Tilbakebetalingstiden og den årlige
kostnaden var utsatt for større endringer, med de laveste verdiene på henholdsvis 16.9 år og
14 329 NOK/år.
Andre resultater i denne oppgaven gav indikasjoner på at varmtvannsforbruket burde være av
en viss størrelse for at bruken av solfangere skal bli lønnsom nok. En tredobling av
varmtvannsbehovet på sykehjemmet i Drammen ga en LCOE på 53.3 øre/kWh. Simuleringer
av varmtvannsbehovet basert på SN/TS 3031 gav grunn til å tro at standarden overestimerer
den beste størrelsen på solfangeranlegg for sykehjem. SN/TS 3031 resulterte i at 50 m2 var den
mest lønnsomme systemstørrelsen. Alle parameterne, med unntak av den årlige kostnaden og
solfraksjonen, oppnådde verre resultater enn forventet fra standarden når det målte
varmtvannsforbruket ble simulert i anlegget på 50 m2. Slike estimeringer av behovet kan gi helt
andre utfall enn forutsett.
Resultatene i denne masteroppgaven får frem viktigheten av økt forskning på bruken av
varmtvann.
Både kostnader
og energibruk
viii
kan minimeres dersom det
faktiske
varmtvannsforbruket i en bygning i hvert tilfelle blir vurdert på forhånd av installasjon av et
solfangeranlegg. Lavere investeringskostnader og/eller økte elektrisitetspriser vil føre til at
solfangere blir et mer attraktivt alternativ.
ix
x
TABLE OF CONTENTS
Preface ………………………………………………………………………………………………………………….. i
Abstract ……………………………………………………………………………………………………………….. iii
Sammendrag………………………………………………………………………………………………………… vii
List of figures ……………………………………………………………………………………………………….. xv
List of tables …………………………………………………………………………………………………………xix
Abbreviations ……………………………………………………………………………………………………….xxi
1
Introduction ………………………………………………………………………………………………………1
2
Theory ……………………………………………………………………………………………………………..5
2.1
Physics ………………………………………………………………………………………………………5
2.1.1
The sun as a resource……………………………………………………………………………..5
2.1.2
Solar radiation ………………………………………………………………………………………7
2.1.3
Thermal conduction ……………………………………………………………………………….8
2.1.4
Thermal convection ……………………………………………………………………………….9
2.1.5
Fluid dynamics …………………………………………………………………………………… 10
2.2
Hot water distribution systems …………………………………………………………………….. 11
2.2.1
Storage heaters …………………………………………………………………………………… 11
2.2.2
Mixing valve ………………………………………………………………………………………13
2.2.3
Circulation system ………………………………………………………………………………. 13
2.3
Solar collectors ………………………………………………………………………………………….13
2.3.1
Types ……………………………………………………………………………………………….. 13
2.3.1.1
Flat plate collector…………………………………………………………………………………….. 14
2.3.1.2
Evacuated tube collector ……………………………………………………………………………. 15
2.3.2
Positioning …………………………………………………………………………………………16
2.3.3
Efficiency …………………………………………………………………………………………..17
2.4
Solar thermal system …………………………………………………………………………………. 21
2.4.1
2.5
Dimensioning ……………………………………………………………………………………..24
Costs ………………………………………………………………………………………………………. 26
xi
2.5.1
Investment costs …………………………………………………………………………………. 26
2.5.2
Operating and maintenance cost …………………………………………………………….28
2.5.3
Electricity cost……………………………………………………………………………………. 28
2.6
3
DHW consumption ……………………………………………………………………………………. 29
2.6.1
The nursing home ……………………………………………………………………………….. 29
2.6.2
SN/TS 3031………………………………………………………………………………………..30
Methods …………………………………………………………………………………………………………. 33
3.1
Analysis of the nursing home data ……………………………………………………………….. 33
3.1.1
3.2
The simulation program – Polysun Designer………………………………………………….. 36
3.2.1
Features …………………………………………………………………………………………….. 36
3.2.2
Settings in Polysun ……………………………………………………………………………… 37
3.3
Chosen DHW system ………………………………………………………………………………… 38
3.4
Assumptions and limitations ……………………………………………………………………….. 40
3.5
Estimation of the investment cost ………………………………………………………………… 41
3.6
Finding the best tilt angle …………………………………………………………………………… 43
3.7
Finding the most profitable solution ……………………………………………………………..44
3.8
Parameters to be represented ………………………………………………………………………. 46
3.8.1
3.9
Economic …………………………………………………………………………………………..46
3.8.1.1
Payback period…………………………………………………………………………………………. 46
3.8.1.2
Annual cost ……………………………………………………………………………………………… 47
3.8.2
4
Conversion of the electrical energy into volume flow ………………………………..33
Technical…………………………………………………………………………………………… 47
Simulations ……………………………………………………………………………………………… 48
3.9.1
Nursing home consumption ………………………………………………………………….. 48
3.9.2
Larger DHW consumption …………………………………………………………………… 48
3.9.3
Consumption based on SN/TS 3031 ………………………………………………………. 48
Results …………………………………………………………………………………………………………… 51
4.1
Results based on the nursing home consumption ……………………………………………. 51
xii
5
4.1.1
Finding the best configuration ………………………………………………………………. 51
4.1.2
The most profitable solution …………………………………………………………………. 53
4.1.3
Larger and smaller accumulator tank ……………………………………………………… 58
4.1.4
Large system ………………………………………………………………………………………61
4.1.5
Small system ………………………………………………………………………………………63
4.1.6
Comparison between system sizes ………………………………………………………….65
4.1.7
Sensitivity analyses …………………………………………………………………………….. 69
4.1.7.1
Investment cost ………………………………………………………………………………………… 69
4.1.7.2
Electricity price ………………………………………………………………………………………… 70
4.2
Larger consumption …………………………………………………………………………………… 72
4.3
Results based on normed inputs from SN/TS 3031 …………………………………………. 75
4.3.1
Finding the best configuration ………………………………………………………………. 75
4.3.2
The most profitable solution …………………………………………………………………. 77
Discussion………………………………………………………………………………………………………. 83
5.1
Assumptions …………………………………………………………………………………………….. 83
5.1.1
Maximum solar collector area and accumulator tank volume ……………………… 83
5.1.2
Irradiation and shading ………………………………………………………………………… 83
5.2
Weather data and DHW consumption ……………………………………………………………84
5.3
System choices ………………………………………………………………………………………….85
5.3.1
Pipes ………………………………………………………………………………………………… 85
5.3.2
Solar collector ……………………………………………………………………………………. 85
5.3.3
Accumulator tank ……………………………………………………………………………….. 85
5.3.4
Solar liquid …………………………………………………………………………………………86
5.3.5
Temperatures………………………………………………………………………………………86
5.3.6
Heat and pressure losses ………………………………………………………………………. 86
5.3.7
Water heater and pumps ………………………………………………………………………. 87
5.4
Methods limitations …………………………………………………………………………………… 88
5.4.1
Result parameters ……………………………………………………………………………….. 88
xiii
5.4.2
Software ……………………………………………………………………………………………. 89
5.4.3
Dimensioning ……………………………………………………………………………………..89
5.4.4
Construction lifetime …………………………………………………………………………… 89
5.5
5.5.1
Installation cost ………………………………………………………………………………….. 90
5.5.2
Accumulator tank cost …………………………………………………………………………. 90
5.5.3
Investment cost ………………………………………………………………………………….. 90
5.5.4
Electricity price ………………………………………………………………………………….. 92
5.6
6
Results discussion ……………………………………………………………………………………..93
5.6.1
SN/TS 3031………………………………………………………………………………………..93
5.6.2
LCOE ……………………………………………………………………………………………….. 93
5.6.3
Payback period …………………………………………………………………………………… 95
5.6.4
Annual cost ………………………………………………………………………………………..96
5.6.5
Area specific collector field yield ………………………………………………………….. 96
5.6.6
Solar fraction ………………………………………………………………………………………97
5.6.7
Maximum collector temperature …………………………………………………………….98
5.6.8
Temperature out of the accumulator tank ………………………………………………… 98
5.6.9
Usefulness ………………………………………………………………………………………….99
Conclusion ……………………………………………………………………………………………………. 101
6.1
7
Choice of costs ………………………………………………………………………………………….90
Further research ………………………………………………………………………………………. 103
References ……………………………………………………………………………………………………. 105
Appendix A: Components in Polysun …………………………………………………………………………..
Appendix B: Controllers in Polysun……………………………………………………………………………..
Appendix C: Specifications of solar collector and accumulator tank ………………………………….
xiv
LIST OF FIGURES
FIGURE 1: The global horizontal solar irradiance of a typical meteorological year (TMY) in
Drammen ……………………………………………………………………………………………………………….6
FIGURE 2: Daily average clear-sky horizontal solar irradiance in March, June, September and
December in Drammen. …………………………………………………………………………………………….6
FIGURE 3: A hot water circulation system, as illustrated in Polysun ……………………………..11
FIGURE 4: Illustrations of series and parallel connection of water heaters …………………….. 12
FIGURE 5: An illustration of a structure of a flat plate solar collector …………………………… 14
FIGURE 6: One type of an evacuated tube solar collector …………………………………………… 15
FIGURE 7: Monthly solar irradiation estimates onto a collector with various tilt angles in
Drammen (59.7°N) ………………………………………………………………………………………………… 16
FIGURE 8: Accumulated irradiation over the year onto collectors with various tilt angles in
Drammen …………………………………………………………………………………………………………….. 17
FIGURE 9: Heat transfer processes in a solar thermal collector ……………………………………. 18
FIGURE 10: Typical flat plate collector efficiencies against a range of temperature differences,
from 0 °C to 100 °C, at various irradiances ………………………………………………………………… 21
FIGURE 11: Solar collector system for hot water preparation as illustrated in Polysun……..22
FIGURE 12: Development of the percentage of the installation costs in relation to the
investment cost ……………………………………………………………………………………………………… 28
FIGURE 13: The yearly energy consumption for DHW at the Drammen nursing home, given
in Wh/m2 ……………………………………………………………………………………………………………… 29
FIGURE 14: Average 24 hours DHW energy consumption profile for the nursing home in
Drammen based on the collected data ……………………………………………………………………….. 30
FIGURE 15: 24 hours DHW energy consumption profile for nursing homes based on the
standard SN/TS 3031. …………………………………………………………………………………………….. 31
FIGURE 16: Average 24 hours DHW energy consumption profile for the nursing home in
Drammen and based on SN/TS 3031 ………………………………………………………………………… 31
FIGURE 17: The yearly variation of energy consumption for DHW at the Drammen nursing
home and based on values from SN/TS 3031. ……………………………………………………………..32
FIGURE 18: Cold water profile over a year, made in Polysun ……………………………………… 35
FIGURE 19: Illustration of the isothermal layers in the tanks in Polysun. ………………………. 36
FIGURE 20: The chosen system diagram as it is shown in Polysun ……………………………….39
FIGURE 21: The accumulator tank price as a function of tank volume ………………………….. 41
xv
FIGURE 22: The accumulator tank price as a function of solar collector area …………………. 42
FIGURE 23: The change in the total investment cost with increasing collector area up to 100
m2……………………………………………………………………………………………………………………….. 42
FIGURE 24: The development of the investment cost per collector area with increasing area
from 10 m2 to 100 m2 …………………………………………………………………………………………….. 43
FIGURE 25: The collector field yield relating to gross area for collector areas of 40 m2 and 80
m2 at different tilt angles, based on the Drammen nursing home consumption …………………. 51
FIGURE 26: The LCOE at different solar collector areas from 10 m2 to 100 m2 based on the
Drammen nursing home consumption ……………………………………………………………………….. 53
FIGURE 27: The solar fraction for each month of the year. …………………………………………. 55
FIGURE 28: The heat energy delivered to the system, divided into the two energy sources –
the solar thermal system and the electric heating element……………………………………………… 55
FIGURE 29: The collector field yield together with the hot water demand in week 1 of 2019
…………………………………………………………………………………………………………………………… 56
FIGURE 30: The collector field yield together with the water temperature out of the
accumulator tank (pipe 1) in week 1 of 2019………………………………………………………………. 56
FIGURE 31: The collector field yield together with the hot water demand in week 26 of 2018
…………………………………………………………………………………………………………………………… 57
FIGURE 32: The collector field yield together with the water temperature out of the
accumulator tank (pipe 1) in week 26 of 2018……………………………………………………………..57
FIGURE 33: The energy flow diagram of the most profitable system …………………………….58
FIGURE 34: The LCOE at accumulator tank volumes of 1000 l, 1500 l, 2000 l, 2500 l and
3000 l …………………………………………………………………………………………………………………..58
FIGURE 35: The payback period at accumulator tank volumes of 1000 l, 1500 l, 2000 l, 2500
l and 3000 l…………………………………………………………………………………………………………… 59
FIGURE 36: The annual cost at accumulator tank volumes of 1000 l, 1500 l, 2000 l, 2500 l
and 3000 l…………………………………………………………………………………………………………….. 59
FIGURE 37: The solar fraction at accumulator tank volumes of 1000 l, 1500 l, 2000 l, 2500 l
and 3000 l…………………………………………………………………………………………………………….. 60
FIGURE 38: The collector field yield relating to gross area at accumulator tank volumes of
1000 l, 1500 l, 2000 l, 2500 l and 3000 l ……………………………………………………………………. 60
FIGURE 39: The maximum collector temperature at accumulator tank volumes of 1000 l, 1500
l, 2000 l, 2500 l and 3000 l ………………………………………………………………………………………61
xvi
FIGURE 40: The LCOE for a system of 80 m2 with tank volumes of 4000 l (50 l/m2 sc), 5000 l
(62.5 l/m2sc) and 6000 l (75 l/m2sc) ……………………………………………………………………………. 62
FIGURE 41: The LCOE for a system of 20 m2 with tank volumes of 1000 l (50 l/m2 sc), 1250 l
(62.5 l/m2sc) and 1500 l (75 l/m2sc) ……………………………………………………………………………. 64
FIGURE 42: The LCOE for system sizes of 20 m2, 40 m2 and 80 m2 ……………………………..66
FIGURE 43: The payback period for system sizes of 20 m2, 40 m2 and 80 m2 ………………… 66
FIGURE 44: The annual cost for system sizes of 20 m2, 40 m2 and 80 m2 ……………………… 67
FIGURE 45: The solar fraction for system sizes of 20 m2, 40 m2 and 80 m2 …………………… 67
FIGURE 46: The collector field yield relating to gross area for system sizes of 20 m2, 40 m2
and 80 m2 …………………………………………………………………………………………………………….. 68
FIGURE 47: The maximum collector temperature for system sizes of 20 m2, 40 m2 and 80 m2
…………………………………………………………………………………………………………………………… 68
FIGURE 48: The LCOE with changes in the investment cost of ± 30 % ………………………… 69
FIGURE 49: The payback period with changes in the investment cost of ± 30 %. …………… 69
FIGURE 50: The annual cost with changes in the investment cost of ± 30 %………………….. 70
FIGURE 51: The LCOE with changes in the electricity price of ± 30 % ………………………… 70
FIGURE 52: The payback period with changes in the electricity price of ± 30 %. …………… 71
FIGURE 53: The annual cost with changes in the electricity price of ± 30 %………………….. 71
FIGURE 54: The LCOE for increasing DHW consumption equivalent to the double and triple
of the nursing home demand ……………………………………………………………………………………. 72
FIGURE 55: The payback period for increasing DHW consumption equivalent to the double
and triple of the nursing home demand ……………………………………………………………………… 72
FIGURE 56: The annual cost for increasing DHW consumption equivalent to the double and
triple of the nursing home demand ……………………………………………………………………………. 73
FIGURE 57: The solar fraction for increasing DHW consumption equivalent to the double and
triple of the nursing home demand ……………………………………………………………………………. 73
FIGURE 58: The collector field yield relating to gross area for increasing DHW consumption
equivalent to the double and triple of the nursing home demand ……………………………………. 74
FIGURE 59: The maximum collector area for increasing DHW consumption equivalent to the
double and triple of the nursing home demand …………………………………………………………….74
FIGURE 60: The collector field yield relating to gross area for collector areas of 40 m2 and 80
m2 at different tilt angles, based on SN/TS 3031 ………………………………………………………….75
FIGURE 61: The LCOE at different solar collector areas from 10 m2 to 100 m2 based on SN/TS
3031 …………………………………………………………………………………………………………………….77
xvii
FIGURE 62: The LCOE of a construction with 50 m2 solar collector area for two different
consumption profiles ……………………………………………………………………………………………… 78
FIGURE 63: The payback period of a construction with 50 m2 solar collector area for two
different consumption profiles …………………………………………………………………………………. 78
FIGURE 64: The annual cost of a construction with 50 m2 solar collector area for two different
consumption profiles. …………………………………………………………………………………………….. 79
FIGURE 65: The solar fraction of a construction with 50 m2 solar collector area for two
different consumption profiles …………………………………………………………………………………. 79
FIGURE 66: The collector field yield relating to gross area of a construction with 50 m2 solar
collector area for two different consumption profiles …………………………………………………… 80
FIGURE 67: The maximum collector temperature of a construction with 50 m2 solar collector
area for two different consumption profiles………………………………………………………………… 80
FIGURE 68: The LCOE as a function of two different pump control mode. …………………… 88
FIGURE 69: Historical electricity prices for industry consumers ………………………………….. 92
APPENDIX FIGURE 1: Screenshot of the settings for the cold-water inlet of the chosen
system in Polysun.
APPENDIX FIGURE 2: Screenshot of the settings for the pipes of the chosen system in
Polysun.
APPENDIX FIGURE 3: Screenshot of the settings for the heat exchanger of the chosen system
in Polysun.
APPENDIX FIGURE 4: Screenshot of the settings for the pumps of the chosen system in
Polysun.
APPENDIX FIGURE 5: Screenshot of the settings for the solar collector of the chosen system
in Polysun.
APPENDIX FIGURE 6: Screenshot of the settings for the tap of the chosen system in Polysun.
APPENDIX FIGURE 7: Screenshot of the settings for the pump controllers of the chosen
system in Polysun.
APPENDIX FIGURE 8: Screenshot of the settings for the heating element controller of the
chosen system in Polysun.
APPENDIX FIGURE 9: Screenshot of the settings for the mixing valve controller of the
chosen system in Polysun.
xviii
LIST OF TABLES
TABLE 1: Unit costs of different components included in a solar thermal construction, based
on SGP Armatec’s products ……………………………………………………………………………………..27
TABLE 2: The annual hot water demand set in Polysun for the two different consumption
profiles ………………………………………………………………………………………………………………… 37
TABLE 3: The different temperatures set in the system to avoid Legionella formation ……..38
TABLE 4: A summary of the numerical values relating to the components in the solar thermal
system ………………………………………………………………………………………………………………….40
TABLE 5: A summary of the chosen economic factors for calculation of the different
parameters ……………………………………………………………………………………………………………. 47
TABLE 6: Collector field yield relating to gross area for a variation of collector areas and tilt
angles, based on the Drammen nursing home consumption …………………………………………… 52
TABLE 7: The LCOE, payback period and annual cost for the most profitable system ……..54
TABLE 8: The solar fraction, collector field yield relating to gross area and maximum collector
temperature over a year for the most profitable system ………………………………………………… 54
TABLE 9: The LCOE, payback period and annual cost for the large system …………………… 63
TABLE 10: The solar fraction, collector field yield relating to gross area and maximum
collector temperature over a year for the large system ………………………………………………….. 63
TABLE 11: The LCOE, payback period and annual cost for the small system ………………… 65
TABLE 12: The solar fraction, collector field yield relating to gross area and maximum
collector temperature over a year for the small system …………………………………………………. 65
TABLE 13: Collector field yield relating to gross area for a variation of collector numbers and
tilt angles, based on SN/TS 3031 ……………………………………………………………………………… 76
APPENDIX TABLE 1: Specifications of the solar collector used in the chosen system.
APPENDIX TABLE 2: Specifications of the accumulator tank used in the chosen system.
xix
xx
ABBREVIATIONS
AM
AR5
DHW
IPCC
KPN
LCOE
NOK
NSF
NTNU
NVE
PU
PVGIS
sc (index)
SN/TS 3031
TMY
Air mass
Fifth Assessment Report
Domestic hot water
The Intergovernmental Panel on Climate Change
Knowledge-Building Project for Industry
Levelised Cost of Energy
Norwegian kroner
The Norwegian Solar Energy Society
Norwegian University of Science and Technology
The Norwegian Water Resources and Energy Directorate
Polyurethane
Photovoltaic Geographical Information System
Solar collector
SN/TS 3031:2016
Typical meteorological year
xxi
xxii
1 INTRODUCTION
Our climate is changing. The latest assessment report (AR5) from the Intergovernmental Panel
on Climate Change (IPCC) states that it is clear that human activity impacts the climate and
that continued “business as usual” will cause long-lasting changes in the environmental system
(IPCC, 2014). In an attempt to mitigate this impact, one of the aims of the Paris Agreement,
which entered into force in November 2016, is to limit the global temperature increase to 1.5
°C above pre-industrial levels (United Nations Framework Convention on Climate Change,
n.d.). Based on this agreement, Norway has legislated several goals concerning greenhouse gas
emissions within the country, including a 40 % decrease in 2030 and a 80-95 % decrease in
2050, both compared to values from 1990 (Klima- og miljødepartementet, 2017).
There exist several areas with potential for improvements in our society, regarding
minimisation of environmental impact. One of them is the energy use in buildings. Several
specifications are found in “Regulations on technical requirements for building works”,
including points on energy efficiency and proscriptions against the use of fossil fuels
(Kommunal- og moderniseringsdepartementet, 2017). In Norway, electricity is the energy
source which is most commonly used in buildings, according to Enova’s statistics from 2017
(Enova, 2019). According to The Norwegian Water Resources and Energy Directorate (NVE),
54 % of the power consumption in buildings was directly connected to the heating of space and
water in 2016 (Spilde et al., 2018). Heat is a form of energy which can be produced by other
means than electricity. This thesis will concentrate on such an alternative energy carrier.
Solar collectors are examples of devices which transform the irradiation from the sun into
thermal energy. In addition to space heating, this energy can warm domestic hot water (DHW).
The energy demand for DHW is much less dependent on the outside temperature than space
heating, and remains approximately constant throughout the year (Andresen, 2008). For this
reason, the extensive solar irradiation in summer can be taken better advantage of. DHW
heating has a share of around 15-20 % of the total energy consumption in Norwegian residential
buildings (SINTEF Byggforsk, 2011). The energy efficiency of buildings will probably increase
in the future. In consequence, the heating of DHW will require a larger part of the building’s
total energy use.
1
In 2017, SINTEF Byggforsk and NTNU initiated the project “Energy for domestic hot water in
the Norwegian low emission society”, in short “VarmtVann2030” (SINTEF Byggforsk, n.d.).
This is a Knowledge-Building Project for Industry (KPN) in cooperation with building owners
and suppliers. Among the reasons for creating this project was the low level of knowledge about
the actual demand of energy for DHW in Norway. Further research on this topic could help
form a basis for future development. The gathering of information about energy use is in
progress and one of the next steps will be to explore possibilities regarding effective and
environmentally friendly solutions. This thesis is a part of the project VarmtVann2030 in
collaboration with SINTEF Byggforsk. The focus will be on the use of solar collectors in
nursing homes.
By the end of 2016, the global capacity of solar thermal collectors in operation was 457 GWth,
71 % of which installed in China, according to the report Solar Heat Worldwide (Weiss &
Spörk-Dür, 2018). Estimations for new installations in 2016 gives 38.3 MWth as the equivalent
value for Norway. The annual energy yield worldwide from water-based solar collectors in
2016 was 375 TWh, giving CO2 savings of 130 million tons. Heating of DHW make up the
largest part of applications, with a share of 94 % of the energy production worldwide in 2016
(Weiss & Spörk-Dür, 2018).
Several studies on the use of solar collectors to heat DHW has been done in Norway. Among
these are reports written by SINTEF which analyse the principles (Andresen, 2008; SINTEF
Byggforsk, 2011) and costs (Skeie et al., 2016) of introducing a solar thermal construction to a
building. In 2015, the Norwegian Solar Energy Society (NSF) and Asplan Viak informed of the
status quo of the use of solar collectors in the country (Norsk Solenergiforening & Asplan Viak,
2015). Statsbygg sponsored a project which, among others, consisted of implementing a solar
collector installation on a student residence building in Evenstad (Selvig et al., 2017). Different
implementations which have been examined in other papers include industrial halls (FidorówKaprawyl & Dudkiewicz, 2017), districts (Fredly, 2014), office buildings (Keul, 2010),
sheltered housing (Larsen et al., 2011), educational facilities (Moratal & Bermejol, 2013) and
single-family buildings (Starakiewicz, 2018). The extensive research show that the use of solar
collectors as energy source is an area of interest and a realistic, provident choice.
Research on an existing solar thermal facility at a hotel in Trondheim, by Aashammer (2016),
gave indications of the actual functioning of the system. There appeared to be a deviation
2
between projected and measured values on the share of contribution from the solar collector
installation of larger than 50 %. Aashammer means that a reason for this could be errors in the
projecting phase of the solar collector construction based on assumptions from the supplier.
SINTEF has written a report on experiences of house owners which has implemented solar
collectors at their residence (Hauge et al., 2014). The report argued, among others, that
improved competence among professionals in the area is necessary. Hence, continued measures
are needed to assure the best possible performance of solar heat installations.
An estimated energy yield potential, given that all of Norway’s residents have a correctly
dimensioned solar collector construction delivering heat to their DHW, is around 5 TWh yearly
(SINTEF Byggforsk, 2011). This amount of energy could replace almost 10 Alta hydro power
plants. The focus in this thesis will be on the possibility of using solar collectors as energy
source for the heating of DHW in nursing homes.
Measurements on DHW energy use has been collected by SINTEF Byggforsk over one year
from a nursing home in Drammen. These values will act as a basis to dimension solar thermal
constructions, using a software called Polysun Designer (Vela Solaris, 2019). Only pressurised
systems with a liquid water solution energy carrier, and in combination with an electric water
heater, will be assessed. The configuration which ends up with the lowest Levelised Cost of
Energy (LCOE) is considered to be the best one. This thesis examines the most profitable solar
collector facility for measured and standard based DHW consumptions in nursing homes, and
changes in various economic and technical parameters with specified alterations.
The issues which will be explored are:
•
What is the most profitable system configuration based on the measured Drammen
nursing home DHW consumption?
•
What characteristics do the most profitable solution have?
•
What changes are noticeable when the accumulator tank size and solar collector area is
altered?
•
How dependent are the economic parameters on alterations in the investment cost and
electricity price?
•
What changes are noticeable on the most profitable system for the Drammen nursing
home when the DHW consumption is increased?
3
•
What is the most profitable system configuration based on the standard (SN/TS 3031)
for nursing homes?
•
What would be the outcomes if the most profitable solution based on SN/TS 3031 was
implemented for the Drammen nursing home DHW consumption?
Chapter 2 will explain the relevant background theory. This includes both physics and
information about solar thermal systems and DHW consumption. Chapter 3 describes the
analysis done, the methods used and the assumptions taken in the research. Chapter 4 represents
the results and objective observations of these. Chapter 5 comprises a discussion of the
assumptions and the results in a broader perspective. Chapter 6 will conclude on the most
important aspects and outcomes in the thesis.
4
2 THEORY
The following chapter will cover the physics relevant to solar collectors. Further, principles of
solar collectors and its system are explained, in addition to a representation of the related costs.
A description of hot water distribution systems is also included. Lastly, the energy
measurements for DHW at the Drammen nursing home and the standard SN/TS 3031 are
depicted.
2.1 PHYSICS
2.1.1 THE SUN AS A RESOURCE
One of our natural energy sources is the sun. The radiation energy from this massive star is in
fact the origin of life on earth (Engvold, 2018). The capacity of the sun is 15 000 times larger
than the earth’s population’s total need for energy (Norsk solenergiforening et al., 2017). This
illustrates the huge potential of the sun as an energy source.
According to NVE (2018), Norway receives between 700 kWh/m2 and 1000 kWh/m2 from the
sun on a horizontal surface each year. At higher latitudes, the intensity is lower because the
same amount of radiation energy is spread over a larger area (Norges vassdrags- og
energidirektorat, 2018). NSF et al. (2017) state that the southeastern part of the country has the
highest potential concerning solar radiation intensity. Naturally, the elevation of the
surroundings and the weather conditions play an additional part on a local perspective. The
solar irradiance is also dependent on the time of the year and day (Norsk solenergiforening et
al., 2017). Figures 1 and 2 show the yearly and daily variation, respectively, in Drammen. The
visualisations are derived from Photovoltaic Geographical Information System (PVGIS), which
is a web application developed at the European Commission Joint Research Centre (European
Commission, 2017). A typical meteorological year (TMY) is a selection of hourly
meteorological data for a given location, based on a time frame of normally 10 years or more
(European Commission, 2019). Each month is represented by data from the most “typical” year
for that month, e.g. January might be from 2010 while July is from 2008 etc.
5
FIGURE 1: The global horizontal solar irradiance of a typical meteorological year (TMY)
in Drammen, based on the years 2006-2017. Source: PVGIS
Daily average irradiance
Daily irradiance [W/m2]
900
800
March
700
June
600
September
500
December
400
300
200
100
0
3
5
7 solar
9 irradiance
11
13
15
17
19
21 year23(TMY)
FIGURE 4: 1The global
horizontal
of a typical
meteorological
Hour PVGIS
in Drammen, based on the years 2006-2017. Source:
FIGURE 2: Daily average clear-sky horizontal solar irradiance in March, June, September
and December in Drammen. Database: PVGIS-SARAH.
It is clear from Figure 1 that summer is the season when the irradiance from the sun is strongest,
with a peak in June. Figure 2 shows that the maximum radiation intensity occur in the middle
of the day and that it is non-existent at night. There are large differences between the months,
both in irradiance peak value and day lenght. Collectively, weather variations form a complex
pattern which might make it difficult to rely on solar radiation as a sole source of energy.
FIGURE 5: The global horizontal solar irradiance of a 6typical meteorological year (TMY)
in Drammen, based on the years 2006-2017. Source: PVGIS
2.1.2 SOLAR RADIATION
All objects having a temperature above absolute zero emit radiant energy and interact with other
objects (Young & Freedman, 2012). Emission describes radiation outwards while absorption,
reflection and transmission are processes referring to the reception of such energy. The
interactions happening on a specific surface depend on properties of both the object and the
radiation (Twidell & Weir, 2006). Total absorptance depends on the different wavelenghts of
the incident radiation and it is the absorbed energy which is considered useful when discussing
solar collectors.
Solar radiation is a form of electromagnetic energy including infrared, visible and ultraviolet
light (Twidell & Weir, 2006). Spending time outside on a sunny day, it is inevitable to feel the
sun’s capability of energy transfer. It is the infrared portion of the radiation we feel as heat
(Hanania et al., 2019). Heat transfer by radiation is a product of interaction between photons in
the radiation and the molecules making up the absorbing body. The molecules move faster,
which in consequence lead to an increase in the internal temperature (Hanania et al., 2017).
This energy transfer will continue until the contributing components reach the same
temperature. The radiation is still present, but the exchange has ceased.
The sun is often considered to be a blackbody, which means that it absorbs all wavelenghts
contained in the radiation which hits it and reflects or transmits nothing. Similarily, it emits
radiant energy comprising a specter of wavelengths dependent on its temperature. This
spectrum is given by Planck’s law and the peak frequency can be derived from Wien’s
Displacement Law. The emissivity, e, of a blackbody is equal to 1. (Twidell & Weir, 2006)
To get a notion of the value of the solar flux, Stefan-Boltzmann law, given below, can be used.
𝑃 = 𝐴𝑒𝜎𝑇 4
(1),
𝑃 = 𝐴𝑒𝜎𝑇 4
where P is the radiated power [W], A is the surface area [m2], e is the emissivity [-], σ = (1),
5.67 ∙
10-8 W/m2K4 is the Stefan-Boltzmann constant and T is the temperature [K]. All the values refer
𝑃 = 𝐴𝑒𝜎𝑇 4
(1),
to the radiating object. (Twidell & Weir, 2006)
𝑃 = 𝐴𝑒𝜎𝑇 4
7
(1),
It is obvious from equation (1) that the radiation power is highly dependent the sun’s
temperature. Taking into consideration the weakening of the radiation from the sun because of
spreading and the distance from earth, the yearly average solar radiation intensity on our planet
is 1367 W/m2 (Amin et al., 2018). The amount of radiation which actually reaches the surface
of the earth depends on cloud cover, particles in the atmosphere and the angle of incidence. For
standarisation purposes, the unit air-mass is defined. At air-mass zero (AM0) the power density
is 1367 W/m2, referring to the solar radiation outside the atmosphere. At AM1.5 it is 1000 W/m2
and this value is typically used as a standard when testing solar technology because it is
considered as “normal” air mass. As a result of radiative interactions in the atmosphere and on
the earth’s surface, there will allways be diffuse radiation, in addition to direct (Twidell & Weir,
2006). Diffuse radiation can for instance be reflection from clouds or windows.
Equation (1) explains the emitted radiation by a body, but it does not show the interaction
between two radiating surfaces. Considering two bodies – 1 and 2 – emitting radiation equally
in all directions and having no absorbing body between them. The net radiative heat flow, Qrad,
from 1 to 2 is then given by:
′
𝑄𝑟𝑎𝑑 = 𝜎(𝑇14 − 𝑇24 )𝐴1 𝐹12
(2),
′
𝑄𝑟𝑎𝑑 = 𝜎(𝑇14 − 𝑇24 )𝐴1 𝐹12
(2),
where σ is the Stefan-Boltzmann constant,
T1 and T2 are the temperatures (in Kelvin) of bodies
1 and 2 respectively, A1 is the surface area of 1 and F’12 is the exchange factor. The exchange
′
𝑄𝑟𝑎𝑑 = 𝜎(𝑇14 − 𝑇24 )𝐴1 𝐹12
(2),on
factor depends on the proportion of the
emitted radiation from 1 which reaches 2, dependent
the geometry of the bodies involved, the area ratio and the emittance of the surfaces. (Twidell
′
𝑄𝑟𝑎𝑑 = 𝜎(𝑇14 − 𝑇24 )𝐴1 𝐹12
(2),
& Weir, 2006)
2.1.3 THERMAL CONDUCTION
Thermal conduction is an essential process in solar collectors, explaining the energy transfer to
the energy carrier. Unlike radiation energy, conduction can only happen between materials
which are in contact. Similar to radiation energy, the warmer object causes vibrations of the
atoms in the colder object. The vibrations spread throughout the medium between atoms and
free electrons and cause a gradual temperature rise. Different materials have different
conduction abilities, denoted thermal conductivity. Metals are for example good conductors
because of their large number of free electrons. (Cooper, n.d.)
8
The equation explaining heat conduction can be given in the form:
𝑄𝑐𝑜𝑛𝑑 = −𝑘𝐴
∆𝑇𝑐𝑜𝑛𝑑
∆𝑥
(3),
(3),
∆𝑇𝑐𝑜𝑛𝑑
where Qcond is the heat transfer rate, 𝑄
k is the=thermal
−𝑘𝐴 conductivity, A is the contact area, ΔTcond
𝑐𝑜𝑛𝑑
∆𝑥
is the temperature difference and Δx is the distance. All the values refer to the connection
(3),
between the surfaces. The minus sign is added to emphasise that the energy flows to the coldest
∆𝑇𝑐𝑜𝑛𝑑
𝑄𝑐𝑜𝑛𝑑 = −𝑘𝐴
place. (Twidell & Weir, 2006)
∆𝑥
(3),
From equation (3) it can be derived that the energy∆𝑇
travels
faster with increased conductivity,
𝑐𝑜𝑛𝑑
𝑄𝑐𝑜𝑛𝑑 = −𝑘𝐴
surface area and temperature difference. Likewise,∆𝑥it slows down with growing distance
between the points of different temperatures.
2.1.4 THERMAL CONVECTION
The last mechanism of heat transfer is convection. This type of energy exchange only happens
to or from a fluid in motion. There are two kinds of convection, natural and forced. When a
fluid is heated, it expands and hence becomes less dense, which in turn makes it rise. This is
natural convection and it is the driving force behind wind on the earth’s surface. Forced
convection is a result of influence from an external impact, for instance an air fan or a water
pump. The initial heating process, before the fluid moves, happens by conduction from a hot
surface. (Twidell & Weir, 2006)
The complexity of the convection process requires simplifications for calculation purposes.
Equation (4) is based on the assumption that the fluid is not moving in the boundary layer. The
boundary layer is the area closest to the heating surface. Hence, an expression for convective
heat transfer, Qconv, can take the following form:
𝑄𝑐𝑜𝑛𝑣 = 𝐴ℎ𝑣 ∆𝑇𝑐𝑜𝑛𝑣
(4),
𝑄
= 𝐴ℎ𝑣 ∆𝑇𝑐𝑜𝑛𝑣
(4),
where A is the cross-sectional area of𝑐𝑜𝑛𝑣
the boundary
layer, hv is the convective heat transfer
coefficient and ΔTconv is the temperature difference across the boundary layer. hv is dependent
(4),
𝑐𝑜𝑛𝑣 = 𝐴ℎ
𝑣 ∆𝑇thermal
𝑐𝑜𝑛𝑣
on the surface shape and fluid flow, in𝑄addition
to the
conductivity of the fluid. (Twidell
& Weir, 2006)
𝑄𝑐𝑜𝑛𝑣 = 𝐴ℎ𝑣 ∆𝑇𝑐𝑜𝑛𝑣
9
(4),
2.1.5 FLUID DYNAMICS
A fluid in motion is called a flow. This thesis will concentrate on liquid flowing in tubes, and
for that reason specific theory regarding other types of flow is excluded. A pipe flow is
physically limited on all sides and is driven by either pressure or gravity. For simplicity, liquids
are usually considered incompressible even though this is not entirely true. (Jones, 2017)
For simplification, a flow is often considered as steady, which means that its properties do not
change with respect to time. Additionally, a flow can be either laminar or turbulent. A laminar
flow is smooth, but not necessarily linear, while a turbulent flow moves anywhere with no
apparent pattern. (Jones, 2017)
The value commonly used to determine whether a flow is laminar or turbulent is Reynolds
number, Re:
𝑅𝑒 =
𝜌𝑣𝑙
𝜇
(5),
(5),
𝜌𝑣𝑙
where ρ is the density, v is the flow velocity,
𝑅𝑒 l=is the characteristic length of the container and
𝜇
μ is the dynamic viscosity (Engineering ToolBox, 2003c). The characteristic length is equal to
(5),
the diameter if the container is a circular tube or duct (Engineering ToolBox, 2003b). The
𝜌𝑣𝑙
velocity relates to the cross section area of𝑅𝑒
the=fluid
𝜇 container (Engineering ToolBox, 2008b).
(5),
With increasing Re, the flow grows in turbulence.
𝜌𝑣𝑙
𝑅𝑒 =
A flow consists of potential, kinetic and pressure𝜇 energy. Losses are unavoidable and in the
case of fluids in motion the majority is due to friction. Considering flow in a tube, these losses
can be expressed by the D’Arcy-Weisbach equation:
∆𝑝𝑙𝑜𝑠𝑠 = 𝜆
𝑙 𝜌𝑣 2
𝑑ℎ 2
(6),
(6),
𝑙 𝜌𝑣 2
where λ is the friction coefficient, l is∆𝑝
the𝑙𝑜𝑠𝑠
length
= 𝜆 of the pipe, dh is the hydraulic diameter, ρ is
𝑑ℎ 2
the fluid density and v is the flow velocity. Equation (6) is valid for a fully-developed, steady
(6),
and incompressible flow (Engineering ToolBox, 2003a). The friction coefficient depends on
𝑙 𝜌𝑣 2
∆𝑝𝑙𝑜𝑠𝑠 = 𝜆
𝑑ℎ 2
(6),
10
∆𝑝𝑙𝑜𝑠𝑠 = 𝜆
𝑙 𝜌𝑣 2
𝑑ℎ 2
the degree of turbulence of the flow and the roughness of the tube surface, and can be found by
solving the Colebrook equation (Engineering ToolBox, 2008a).
2.2 HOT WATER DISTRIBUTION SYSTEMS
A hot water distribution system consists of one or multiple water heaters, a piping system,
valves and the tap (Zijdemans, 2014). Figure 3 displays a possible structure. There are different
types of water heaters, mainly divided into flow heaters and storage heaters. In flow heaters,
the water is heated at the tap, while storage heaters accumulate hot water in a tank. In this thesis,
the focus will be on storage heaters since they provide inertia in the system.
FIGURE 3: A hot water circulation system, as illustrated in
Polysun. This comprises a direct storage heater. Used with kind
permission from Vela Solaris.
2.2.1 STORAGE HEATERS
Storage heaters can be separated into direct and indirect types. Direct heating happens by means
of an electric heating element which is placed inside the tank in contact with the water. Due to
low installation and electricity costs, this is the most common type of storage heaters in Norway.
In standard direct storage heaters, the heating element is normally located in the bottom of the
tank to heat its entire contents, as in figure 3. Fast heaters also exist, in which the top tank water
is heated first to ensure that the water in the tap becomes hot as quickly as possible. (Zijdemans,
2014)
11
FIGURE 7: A hot water circulation system, as illustrated in
An indirect storage heater usually contains a spiral pipe (coil) or an external heat exchanger.
The coil is placed inside the tank and holds a medium of higher temperature than the
accumulated water, thereby heating it up. Coil heaters are mostly used in large facilities and
may be part of combination systems. The external heat exchanger is commonly of the plate
type. A circulation pump carries the cold water from the bottom of the tank, through the heat
exchanger and delivers the heated water to the top. Indirect and direct storage heaters can be
combined, for instance by achieving a certain temperature indirectly and using an electric
heating element for reheating. There are several energy sources which can provide heat energy
to such a system, e.g. heat pumps, bio energy or solar collectors. (Zijdemans, 2014)
For larger buildings it might be necessary to connect multiple water tanks (Zijdemans, 2014).
In principle, there are two ways of doing this: series and parallel. In a series connection (to the
left in figure 4), the water flows through each tank in turn and this solution works well with an
external heat exchanger. This is also a suitable method to attain good temperature stratification.
Water density changes with temperature and this makes the hotter part lighter (Norsk
solenergiforening et al., 2017). As a consequence, the warm water will lie as a layer on top of
the colder in a tank. Thermal stratification is the division of these layers. When the tanks are
connected in parallel (to the right in figure 4), the water is divided equally into each tank
(Zijdemans, 2014). In direct storage heaters, a parallel connection might be beneficial because
all the heating elements will kick in at the same time, resulting in a shorter heating time and
even use.
FIGURE 4: Illustrations of series and parallel connection of water heaters (Fuchs, 2013).
12
2.2.2 MIXING VALVE
The temperature in an electrically heated water tank is often set to be higher than the desired
tap temperature. The reason for this is to achieve a large energy capacity in the tank. Between
the tank and the tap, the hot water is mixed with cold water by means of a mixing valve,
illustrated in figure 3. The mixing valve can function both thermally and mechanically. The
thermally based types continually adjust the mix ratio to get a given temperature. In the
mechanical mixing valves, the ratio between hot and cold water is constant. (Zijdemans, 2014)
2.2.3 CIRCULATION SYSTEM
After periods of no use, there might take some time before the hot water reaches the tap because
the water lying in the pipes has been cooled. This leads to excessive use of water. To avoid such
water waste and waiting time, a circulation system may be applied. A circulation system
consists of pipes which lead the heated water in a circuit between the tap and the mixing valve.
This prevents the water from being still over a longer period. The system is dimensioned in
order to always circulate a given amount of water, and there exist several variants. Heat losses
increase with higher water temperatures, such as in a circulation system. The size of these losses
is dependent on the length of the pipes, which is usually largest from the water heater to the tap.
This results in more heat losses with circulation than without. An alternative to a circulation
system is to install heat tracers alongside the pipes, underneath the insulation layer. (Larsen,
2014)
2.3 SOLAR COLLECTORS
A solar collector transforms radiation energy from the sun into heat, which again is transferred
to a carrier, usually a liquid. The most important component in a collector is the absorber, as it
is the part which does the work of energy transfer. The absorber should have good absorption
characteristics and is therefore often coloured black. A selective coating further increases the
absorber performance by decreasing the emittance of infrared radiation. This coating usually
have an absorbance of around 98 %. (SINTEF Byggforsk, 2011)
2.3.1 TYPES
Among liquid based solar collectors there are two types which are most widely used: flat plate
and evacuated tube.
13
2.3.1.1
F LAT PLATE COLLECTOR
By the end of 2016, 83 % of the total capacity of solar collectors in Europa was of the type flat
plate (Weiss & Spörk-Dür, 2018). Flat plate collectors also form the majority in Norway (Norsk
solenergiforening et al., 2017). The basic design consists of a plane absorber, channels or tubes
for the heat carrier, glazing and insulation, illustrated in figure 5 (Alternative Energy Tutorials,
2019b). The glazing is added to increase the temperature in the collector (SINTEF Byggforsk,
2011). It is made of a transparent material (e.g. glass or plastic) and keeps the heat inside by
admitting shortwave radiation but hindering the longwave radiation from escaping, like a
greenhouse. Insulation in the bottom and on the sides decreases the heat loss even more,
especially the conduction losses (Norsk solenergiforening et al., 2017). The absorber plate is
usually framed in aluminium.
FIGURE 5: An illustration of a structure of a flat plate solar collector. The glazing,
pipes, absorber and insulation is shown. The cold water enters at the bottom and the
flow divides between the pipes. On its way to the top it is heated and the flows meet
again before leaving the collector with a warmer temperature. The figure is used with
kind permission from Alternative Energy Tutorials (2019b).
The absorber can be made of aluminium, copper or plastic (polymer). The choice of material
depends on the type of system. Solar collectors with metal absorbers deliver higher
temperatures and has a higher efficiency compared to those with polymer absorbers. The heat
medium is often contained in tubes which are welded on the backside of the absorber. (Norsk
solenergiforening et al., 2017)
14
FIGURE 13: An illustration of a structure of a flat plate solar collector. The glazing,
2.3.1.2
E VACUATED TUBE COLLECTOR
In China, 92 % of the solar thermal capacity was provided by evacuated tube solar collectors
by the end of 2016. The equivalent number in Europe was 14 % (Weiss & Spörk-Dür, 2018).
Evacuated tube collectors consist of two-layered glass cylinders with vacuum in between
(Alternative Energy Tutorials, 2019a). The vacuum works as an insulator against convection
and radiation losses to the surroundings. The absorber, an aluminium or copper sheet, is placed
inside the inner tube and connected to a metal pipe containing the liquid, as can be seen in figure
6.
FIGURE 6: One type of an evacuated tube solar collector, consisting of four tubes
with absorber plates and heat pipes. On the top there is a heat exchanger in which
the solar liquid flows. The figure is used with kind permission from Alternative
Energy Tutorials (2019a).
There are some different possibilities regarding the construction and functioning of evacuated
tube collectors (Alternative Energy Tutorials, 2019a). For instance, the heat carrier can flow
directly through the tube in a U-bend (direct flow) or it can receive energy by the use of a heat
exchanger (heat pipe). The degree of flexibility and efficiency are among the properties which
vary with choice of configuration.
Several individual tubes are connected via a manifold to form the collector. Because of the
cylindrical shape, evacuated tube collectors have the benefit of always receiving sunlight
perpendicularly independent of its position on the sky. Large irradiation angles, leading to a
high degree of reflection, is a common problem with flat plate solar collectors. The benefit of
15
FIGURE 16: One type of an evacuated tube solar collector, consisting of four tubes
with absorber plates and heat pipes. On the top there is a heat exchanger in which
the round shape together with excellent insulation characteristics, results in the production of
high temperatures and a good overall efficiency. However, due to the vacuum insulation,
evacuated tube collectors are prone to overheating. (Alternative Energy Tutorials, 2019a)
2.3.2 POSITIONING
In addition to the location of the solar collector, orientation and tilt angle are also important
aspects. How to position a collector is dependent on the specific need (Norsk solenergiforening
et al., 2017). An orientation towards south is optimal to take the most advantage of insolation,
but deviations of less than 45° does not affect the energy yield considerably (SINTEF
Byggforsk, 2011). Having a relatively low sun in Norway, the tilt angle should be quite steep.
An inclination of 10° below the latitude of the location is a general rule for estimation purposes.
NSF (2017) propose a collector slope of around 45° for a solar collector which will be used for
heating DHW exclusively. Figure 7 shows the effect of tilt angle on solar irradiation over the
year, based on data from Meteonorm via Polysun.
Solar irradiation over the year
Monthly irradiation [kWh/m2]
180
160
0°
140
30°
120
45°
100
60°
80
90°
60
40
20
0
FIGURE 7: Monthly solar irradiation estimates onto a collector with various tilt angles in
Drammen (59.7°N). The graphs are based on data from Meteonorm.
In summer, a 30° inclination recieves most radiation, while during autumn, winter and spring,
a larger angle can seem to be best in this regard. It is clear that the hight of the sun affects the
optimal tilt angle of a collector. For this reason, it is beneficial to define the use as detailed as
possible before installing a solar thermal construction. To get a notion of what the best choice
of collector slope would be throughout the year, the monthly values are added and shown in
16
figure 8. An inclination of 45° receive most irradiation on a yearly basis, which corresponds
well with both the general rule and NSF’s suggestion.
Accumulated yearly values of solar irradiation
Yearly irradiation [kWh/m2]
1200
1035.5
1064.7
1032.5
1000
806.5
799.8
800
600
400
200
0
0°
30°
45°
60°
90°
Tilt angle
FIGURE 8: Accumulated irradiation over the year onto collectors with various tilt
angles in Drammen. The graphs are based on data from Meteonorm.
2.3.3 EFFICIENCY
The efficiency of a solar collector describes how well it is able to utilise the energy it receives
from the sun. There are several means of heat losses in a collector, dependent both on material
properties and external conditions. The main parts of these losses are due to radiation and
convection. Figure 9 shows the processes happening in a flat plate solar collector. For
simplification, only flat plate collectors will be considered in this chapter. (Quaschning, 2004)
FIGURE 22: Accumulated irradiation over the year onto collectors with various tilt
angles in Drammen. The graphs are based on data from Meteonorm.
17
conduction
conduction
conduction
FIGURE 9: Heat transfer processes in a solar thermal collector. The irradiance
from the sun is both transmitted and reflected when striking the glazing. Inside
conduction
the frame it reflects some of the outgoing infrared radiation from the absorber
while another part is let out. Additionally, there are both radiation and
conduction losses from the absorber. The heat remaining after the losses is
denounced as available heat, represented by the green arrow. The figure is used
with kind permission from Volker Quaschning (2004).
Solar radiation hits the collector in both direct and diffuse form. Reaching the glazing, the
majority of the radiation is transmitted, but some of it is also reflected. The amount which is
reflected vary with the angle of incidence of the sun, as illustrated in figure 7. The
characteristics of the absorber defines how much heat it is able to receive and keep, a selective
coating acquiring the highest amounts. The reflective properties of the glazing has a positive
effect as well, trapping the emitted heat radiation from the absorber inside the collector.
However, a part of the emittance also manages to leave through the cover. (Quaschning, 2004)
The difficulty of completely air sealing the collector makes it exposed to convection losses.
These losses will be highly dependent on the prevailing weather conditions. Strong winds will
for instance maximise the losses because the heated air masses will continually be removed.
FIGURE 25: Heat transfer processes in a solar thermal collector. The irradiance
(Quaschning, 2004)
from the sun is both transmitted and reflected when striking the glazing. Inside
the frame it reflects some of the outgoing infrared radiation from the absorber
Inwhile
a flatanother
plate solar
radiant power
captivated
by the and
absorber, Pabs, is given by
part collector,
is let out. the
Additionally,
there are
both radiation
conduction losses from the absorber. The heat remaining after the losses is
denounced as available heat, represented by the green arrow. The figure is used
with kind permission from Volker Quaschning (2004).
18
𝑃𝑎𝑏𝑠 = 𝜏𝑐𝑜𝑣 𝛼𝑝 𝐴𝑝 𝐸
(7),
where τcov is the transmittance of the glazing
𝑃𝑎𝑏𝑠 =(cover),
𝜏𝑐𝑜𝑣 𝛼𝑝α𝐴p𝑝is𝐸 the absorbance of the absorber (plate),
(7),
Ap is the area of the absorber and E is the irradiance at the absorber. τ, α and A are usually
specified for a given collector while E𝑃can
be 𝜏measured.
𝑎𝑏𝑠 =
𝑐𝑜𝑣 𝛼𝑝 𝐴𝑝 𝐸(Twidell & Weir, 2006)
(7),
To account for the heat losses, a simplified
is used. All the three forms of(7),
heat
𝑃𝑎𝑏𝑠 = 𝜏expression
𝑐𝑜𝑣 𝛼𝑝 𝐴𝑝 𝐸
transfer – conduction, convection and radiation – are dependent on the temperature difference
between the two bodies participating in the process, as can be seen in equations (2), (3) and (4).
Each of these transfer forms have a thermal resistance related to it:
𝑅𝑐𝑜𝑛𝑑 =
∆𝑥
𝑘 𝑝 𝐴𝑝
(8),
(8),the
∆𝑥 distance between the absorber and
Rcond is the conductive resistance where Δx is the
𝑅𝑐𝑜𝑛𝑑 =
𝑘𝑝 𝐴𝑝 conductivity of the absorber plate and
surroundings of different temperature, k is the thermal
p
Ap is the absorber area. (Twidell & Weir, 2006)
(8),
∆𝑥
𝑘 𝑝 𝐴𝑝
(8),
1
(9),
𝑅𝑐𝑜𝑛𝑣 =
ℎ𝑣 𝐴
∆𝑥
𝑅𝑐𝑜𝑛𝑑 =
𝑘 𝑝 𝐴𝑝
(9),
Rconv is the convective resistance where hv is the convective
heat transfer coefficient and A is
1
𝑅𝑐𝑜𝑛𝑣 =
ℎ𝑣 𝐴 & Weir, 2006)
the cross-sectional area of the boundary layer. (Twidell
(9),
𝑅𝑐𝑜𝑛𝑑 =
11
𝑅𝑟𝑎𝑑 𝑅≈𝑐𝑜𝑛𝑣 = ℎ 𝐴
4𝜎𝐴𝑝 𝐹′𝑣𝑝𝑎 𝑇𝑎𝑣 3
(10),
(9),
(10),
1
1
𝑅𝑐𝑜𝑛𝑣 =
Rrad is an estimation of the thermal𝑅resistance
forℎ𝑣radiation
𝑟𝑎𝑑 ≈
𝐴
3 when (Tp-Ta)