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Thermal and economic comparisons of solar heating systems with seasonal storage used in building heating
ARTICLE IN PRESS Renewable Energy 33 (2008) 2532– 2539
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal and economic comparisons of solar heating systems with seasonal storage used in building heating Aynur Ucar , Mustafa Inalli Department of Mechanical Engineering, Firat University, 23279 Elazıg˘, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 September 2007 Accepted 22 February 2008 Available online 9 April 2008
In this study, the thermal performances and economic savings of the three types of central solar heating system with seasonal storage are compared. Three types of seasonal storage were simulated: storage tank without insulation on ground, storage tank with insulation on ground, and underground storage tank without insulation. The long-term temperatures of water in the storage tank are calculated by finite element code ANSYSTM. The simulation results showed that the higher solar fraction and savings are obtained for system with storage buried into ground. Furthermore, the solar fraction of the storage tank system with insulation is significantly higher than that of without insulation storage system. Also, the solar fraction and savings of system with the evacuated tube collector are higher compared to other black paint flat plate collector. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Seasonal storage Building heating Solar energy
1. Introduction The rapidly increasing world population growth gives rise to a greatly increased demand for energy. To maintain the standard of living in industrialized countries and to improve the situation in developing countries, energy consumption must be much more efficiently utilized and energy supply includes a higher share from renewable sources. The store of solar heat from the summer to the winter for space heating is important because of large differences between solar energy supply and heat demand. Nordell and Hellstro¨m [1] evaluated the performance of a solar-heated low temperature space-heating system with seasonal storage in the ground using the simulation models TRNSYS and MINSUN together with the ground storage module DST. They developed an economically feasible design for a total annual heat demand of about 2500 MWh. It was found that total annual cost of the solar heating system was reduced by about 20% to about 125.6792$/MWh, which was lower than the best conventional alternative. Breger et al. [2] made a comparative analysis of the heat transfer from boreholes and U-tubes using analytical solutions, finite element modeling and the available simulation model. This analysis is used to support the development of a methodology by which the heat transfer of any U-tube configuration can be modeled by appropriately specifying parameters in the borehole storage simulation model. A central solar plant with
Abbreviations: TRNSYS, transient system simulation program; CSHPSS, central solar heating plant with seasonal storage; MINSUN, simulation program of central solar heating system. Corresponding author. Tel.: +90 0424 2370000, +90 0424 2375233. E-mail addresses: aucar@firat.edu.tr (A. Ucar), minalli@firat.edu.tr (M. Inalli). 0960-1481/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.02.019
seasonal storage (CSHPSS), which is under construction in Korea, is simulated using TRNSYS to predict thermal performances and economic aspects by Chung et al. [3]. Their system consists of two arrays of collectors, a medium-sized storage tank and two thermal loads. It was found that TRNSYS prediction for system is that about 39% of the total heating load of 885,000 MJ/year can be provided from the sun. Pahud [4] analyzed a central solar heating plant with seasonal ground storage by dynamic system simulations. He is investigated the thermal performance of the system consist of a collector area, water buffer storage and duct storage in the ground for different thermal loads. Reuss et al. [5] investigated the thermal performance of duct systems with vertical heat exchangers. Thermal performance of these systems is influenced by the heat and moisture movement in the area surrounding the heat exchangers. In this study, the combined heat and moisture transport was simulated on the computer for temperatures up to 90 1C. The design data of an interseasonal storage system in Greece are compared with the results of two CSHPSS simulation software codes, MINSUN and SOLCHIPS by Argiriou [6]. The comparison showed good agreement and therefore these userfriendly tools can be used with confidence for the design of CSHPSS under Greek weather conditions. In authors’ previous work [7], thermal performance and economic feasibility of two types of central solar heating system with underground seasonal storage under four climatically different Turkey locations were investigated. The effects of storage volume and collector area on the thermal performance and cost were studied for three load sizes. The temperature distribution in the ground was solved by using finite element method and finite element code ANSYSTM is chosen as a convenient tool. In the authors’ work [8], an exergoeconomic model was developed for
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
Nomenclature Ac Atpk CA CE CF df F I i MS N Np Qal
collector area, m2 ground contact area, m2 area dependent cost, $/m2 fixed cost, $ cost of energy from fuel, $/GJ down payment solar fraction monthly average daily solar radiation incident on the collector per unit area, MJ/m2 day assumed annual interest rate on mortgage solar system performance degrade period of economic analysis, year payback period of system, year design house heat load, W
analysis and optimization of solar heating systems with residential buildings. The optimum collector area (Ac) and storage volume (V) for solar assisted heating system in the Elazıg˘, Turkey (38.71N), by using MATLAB optimization toolbox were obtained. The energy and energy loss in each of the components of a solar heating system with seasonal storage were also determined. The most promising source of energy for Turkey is solar energy because of its climate. However, solar thermal is only used directly to a small extent. Therefore, the use of solar energy for residential building heating has become very important for the Turkish economy. For the economic design of the central solar heating system with seasonal storage, a theoretical study is necessary before installation. In this study, we compare the performance of the three types of central solar heating system with seasonal storage. The systems that can be simulated are: storage tank without insulation on ground, storage tank with insulation on ground and underground storage tank without insulation. The thermal performances of flat plate collectors and evacuated tube collectors are calculated in this study.
QB QS Qu Rv S (UA)B (UA)strg Ta Th Tcol Tiref Toref Ts W Z
heat load of building, W heat loss from the storage tank, W monthly average of useful solar energy, W resale value of solar system, $ solar savings, $ building loss coefficient, W/K storage tank loss coefficient, W/K ambient temperature, K temperature of fluid in heat exchanger, K collector temperature, K inside design air temperature, K winter design outside air temperature, K fluid temperature at the inlet to the collector, K heat pump power collector efficiency
where Ac is the collector area, I is the instantaneous solar radiation incident on the collector per unit area. Thermal performances of the black paint flat plate and vacuum tube collectors are researched in this study. The collector efficiency for the evacuated tube collector is given by Zeva ¼ 0:84 2:02ðT m T a Þ=I 0:0046I½ðT m T a Þ=I2
Zflat ¼ 0:70 3:4ðT m T a Þ=I
(3)
Efficiency curves for the evacuated tube and flat plate collectors are plotted in Fig. 2. It can be shown that the evacuated tube collector efficiencies are much better than those of the glazed flat plate collectors for the higher values of the temperature difference between absorber and ambient air.
A schematic diagram for the system consisting of plate solar collectors, a heat pump, a storage tank, and heat load is shown in Fig. 1. The heat produced by the collectors throughout the year is stored in the storage tank. This heat is used to provide house heating during the heating season. In the following paragraphs, modeling of the system components, including load and the method of solution are given. 2.1. Solar collectors The useful energy gain of collector surface area is given by the following equation [9]: (1)
(2)
where Tm is the mean collector temperature and Ta is the ambient air temperature. The efficiency for the flat plate collector is calculated by the following equation:
2. The thermal system
Q u ¼ ZAc I
2533
Fig. 2. Efficiency curves of vacuum tube and flat plate collectors.
Fig. 1. Schematic diagram of a solar heating system with storage tank on ground.
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A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
2.2. Storage tank
2.3. Heating loads
In this study, a cylindrical tank is selected as a storage tank type and is an important part of a solar heating system. Selected glass wool is the insulation material of the tank which is placed on the ground. The heat loss from the tank on ground is calculated by
Solar energy is supplied from the storage tank to a building and instantaneous heat load for building is evaluated using the following equation:
Q s ¼ ðUAÞstrg ðT col T a Þ
A1 þ ðUAÞgnd ðk1 =d1 Þ þ ð1=hÞ
(5)
where k1 and d1 are the thermal conductivity and thickness of insulation and h is the convective heat transfer coefficient. In this study, the conduction resistance of the tank wall is neglected because the wall thickness is very thin. (UA)gnd represents heat loss through ground from the bottom of the tank and calculated using the following relation [3]: ðUAÞgnd ¼
Atpk ðd2 =k2 Þ þ ð4r=3prkgnd Þ
if d2 40:2r
k2 kgnd
(7)
The heat loss coefficient (UA)B of the building is calculated as
(4)
where (UA)strg is the loss coefficient of storage tank and calculated using the following equation [3]: ðUAÞstrg ¼
Q B ¼ ðUAÞB ðT iref T a Þ
ðUAÞB ¼ Q al =ðT iref T oref Þ
(8)
where Qal is the annual heating load of the building. If the energy demand of the building is provided by a heat pump, the instantaneous heat load of the building is Q B ¼ WðCOPÞ ¼ Wbp T h =½T h T s
(9)
where bp is the characteristic coefficient of the heat pump, which is in the range of 0.2–0.3. The solar fraction is defined as the utilized solar heat divided by the total heat demand and calculated from the following relationship: X F¼ ½Q u Q s =Q B (10)
(6) 3. Economic analysis
where k2 and d2 are the thermal conductivity and thickness of insulation on the tank bottom and kgnd is the thermal conductivity of the ground.
The economic analysis of solar heating system performed using the simplified P1 and P2 method [9]. Solar savings S, are given by S ¼ P1 C F Q B F P2 C S
Table 1 Simulation parameters of the storage tank and load
(11)
where CS is calculated as follows: C S ¼ C A Ac þ C E Parameters
Storage tank Type Insulation material Insulation thickness Insulation thermal conductivity
Storage tank on ground Glass wool 0.5 m 0.026 W/m K
Load Design house heat load, Qal Building loss coefficient, (UA)B Inside design air temperature, Tiref Winter design outside air temperature, Toref
315360 MJ/year (per house) 9556 MJ/year K (per house) 294 K 261 K
(12)
where CA is the total area dependent co‘ts for the solar system ($/m2). These costs include the cost of the collector and part of the cost of the thermal storage equipment. CE is the total cost ($) of the equipment, which is independent of the collector area. Examples are costs for piping or ducts, controls, blowers, and so on. P1 and P2 are calculated by P1 ¼ ½ð1 þ df Þ=ðdf iÞ½1 ðð1 þ iÞ=ð1 þ df ÞÞN
(13)
P2 ¼ 1 þ P 1 M S Rv ð1 þ df ÞN
(14)
The payback period Np is Np ¼ ln½1 ðP 2 C S =C F Q B FÞðdf iÞ=ð1 þ df Þ= ln½ð1 þ iÞ=ð1 þ df Þ
Table 2 Economic parameters
(15)
Type
Collector types
Area dependent cost, CA (per house) Fixed cost, CE (per house) Cost of energy from fuel, CF Solar system performance degrade, MS Down payment, df Assumed annual interest rate on mortgage, I Resale value of solar system, Rv (per house) Period of economic analysis, N
4. Simulation parameters
Flat plate
Evacuated tube
1200$/m2 4000$ 50$/GJ 1% 5% 5% 4000$ 50 years
1628$/m2 4430$ 50$/GJ 1% 5% 5% 4430$ 50 years
In this study, water temperature in storage tank of the solar system is calculated by finite element code ANSYSTM. The monthly outside air temperature and net energy input rate in the storage tank were transferred to ANSYS as the input data and monthly average water temperature in the storage is found for the periodic operation regime. The solar systems that can be simulated are flat plate collectors and evacuated tube collectors. The load size considered was 25,
Table 3 Monthly weather data for Elazıg˘ [10]
Ta (1C) I (MJ/m2/day)
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jun.
27.2 18.3
27.0 19.2
22.0 17.5
14.8 13.5
7.8 8.5
1.5 6.9
1.3 7.1
0.0 10.7
4.7 12.6
11.8 15.4
17.4 15.6
22.9 18.1
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
250, and 1000 house units. The systems that can be simulated are: storage tank on ground (without insulation), storage tank on ground (with insulation), and underground storage tank (without
Average temperature (°C)
120
Underground storage tank (without insulation) Storage tank on ground (with insulation) Storage tank on ground (without insulation)
100 80 60
2535
insulation). The detailed data required for simulation of these three seasonal storage types and heating load are given in Table 1. The economic analysis of solar heating system performed by using the simplified P1 and P2 method and parameter values used in the economic analysis are given in Table 2. Simulations were performed for Elazıg˘ location in Turkey, which is at latitudes between 36–421N. The weather data used in the simulation are average values based on many years of measurement in meteorological stations in Turkey. The data for the monthly average solar radiation on a horizontal surface and the monthly average outside air temperature of this location are given in Table 3.
40
5. Results and discussion
20 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 Time (year)
Fig. 3. Temperature variation water in storage tank during a simulation.
The results in this study are valid for a periodic operation case of the system. Fig. 3 shows water temperature variation in storage tank over years. In order to reach the periodic operation regime, the system with underground storage tank requires approximately 23 years. When a storage tank without insulation on
Fig. 4. Solar fraction as a function solar collector area for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
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A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
ground is used, required time to reach periodic operation regime of the system is 5 years. The maximum storage tank temperature for this system is set to 40 1C, while it is 100 1C for the system with underground storage tank. It can be seen that water temperature difference between the systems with insulation and without insulation on ground is 50 1C. The solar fraction as a function of solar collector area for three loads calculated using the weather data for Elazıg˘ are given in Fig. 4. The solar fraction increases with increase of solar collector area for three seasonal storage types. The heat loss from the storage in underground storage system is less than that of the system with storage tank on ground and therefore a smaller collector area is required in order to achieve higher solar fraction value. It is observed that when storage tank with insulation on ground is used, the collector area required in order to achieve 0.70 solar fraction is 22 m2/house, i.e. 9% bigger than that of the underground storage tank system. Fig. 5 shows the solar fraction as a function of storage volume for three loads. The solar fraction increases with increasing storage volume. The higher solar fraction takes place for the
system with underground storage tank, compared with the other two storage systems. Using same storage volume (80 m3/house), solar fraction value is reached to 0.72 for the storage tank with insulated on ground and it is 0.60 for storage system without insulation on ground. The solar saving as a function of solar collector area for three loads is given in Fig. 6. The solar saving of system increases with increase of solar collector area for three types of storage. It can be seen that saving of the system with underground storage tank is higher than that of the other two storage systems. In the case of the 250 house loads and 20 m2/house collector area, saving of underground storage tank system is 15% higher than that of storage tank with insulation on ground and 47% than of noninsulated storage tank on ground. Fig. 7 shows the solar saving as a function of storage volume for three loads. The solar fraction increases with increasing storage volume and therefore the savings of solar system increases depends on storage volume. The higher solar saving takes place for the system with underground storage tank, compared with the other two storage systems.
Fig. 5. Solar fraction as a function storage volume for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
Savings ($)
2.0E+07
2.5E+08
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
2.0E+08 Savings ($)
2.4E+07
1.6E+07 1.2E+07 8.0E+06
2537
1.5E+08 1.0E+08 5.0E+07
4.0E+06
0.0E+00
0.0E+00 10
15
20
25
10
30
15
1.0E+09
Savings ($)
8.0E+08
20
25
30
Collector area (m2/house)
Collector area (m2/house) Storage tank on ground (without insulation) Storage tank on ground (without insulation) Storage tank on ground (without insulation)
6.0E+08 4.0E+08 2.0E+08 0.0E+00 10
15
20
25
30
Collector area (m2/house) Fig. 6. Solar saving as a function solar collector area for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
2.5E+07
1.5E+07
2.5E+08
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
2.0E+08 Savings ($)
Savings ($)
2.0E+07
1.0E+07 5.0E+06
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
1.5E+08 1.0E+08 5.0E+07
0.0E+00
0.0E+00 40
60
80
100
120
40
Storage volume (m3/house) 1.1E+09
Savings ($)
9.0E+08
60
80
100
120
Storage volume (m3/house)
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
7.0E+08 5.0E+08 3.0E+08 1.0E+08 40
60
80
100
120
Storage volume (m3/house)
Fig. 7. Solar saving as a function of storage volume for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
The effect of the load size upon the solar saving is illustrated in Fig. 8. This figure shows that the saving increases with the increase of the house load. It can be seen that the saving for 1000 house loads is 63% higher than that of 500 house loads. Fig. 9 shows effect of collector efficiency on solar fraction and savings for the three load sizes. The solar fraction and savings of systems with the evacuated tube collector which has an evacuated absorber strip are higher, compared to other black paint flat plate collectors. It is observed that the required surface area for the flat plate collectors is larger than that of evacuated tube collectors. For example, the required collector area to achieve a solar fraction of 70% for 25 house loads is 18 m2 for evacuated tube collector and 20 m2 for the flat plate collector. The payback period and solar fraction for three locations are given in Table 4. The payback period was evaluated
by using a system lifetime of 50 years. These results show that the payback period of system for the tank with insulation on ground is 10% higher than that of a system with underground storage tank. The minimum payback period of system with underground storage tank is obtained as 19 years. For 20 m2 collector area, the payback period of system for the tank without insulation on ground is 42 years, while for underground storage tank it is 23 years. The performance of a solar heating system with seasonal storage is directly related to the water temperature of the storage tank. The effect of ambient temperature on the water temperature in storage for system with underground storage tank is less than of the other two storage systems. Therefore, the lowest payback period occurs with the system with underground storage tank.
ARTICLE IN PRESS 2538
A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
6. Conclusions
solar collector efficiency on performance of system is discussed. The following items can be summarized from these analyses:
In the present study, thermal performance and economic feasibility of the three types of central solar heating system with seasonal storage in Turkey is investigated. In addition, the effect of
(a) The higher solar fraction is obtained for system with storage buried inside ground for three load sizes. The solar fraction increases with increase of solar collector area for three seasonal storage types. (b) It is shown that the solar saving of the system with underground storage tank is higher than that of the other two storage systems. The heat loss from a tank without insulation which is placed on the ground is higher than that of tank with insulation and therefore savings of this system are much higher.
Table 4 Payback period and solar fraction for the three systems (25 houses, V ¼ 40 m3/ house)
Fig. 8. Solar savings of the storage tank system with insulation on ground for three load sizes.
Collector Storage tank on ground (without insulation) area, A (m2/ Payback Solar house) fraction, period, N (years) F
Storage tank on ground Underground storage (with insulation) tank Payback Solar fraction, period, N (years) F
Payback Solar fraction, period, N (years) F
10 15 20 25 30
0.30 0.46 0.61 0.76 0.90
0.32 0.49 0.67 0.81 0.97
0.18 0.27 0.37 0.47 0.57
34 40 42 43 43
Fig. 9. Effect of collector efficiency on solar fraction and savings for the three loads in Elazıg˘.
21 24 25 26 27
19 22 23 25 25
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
(c) The required storage volume to achieve a certain solar fraction for storage tank with without insulation on ground is bigger than that of storage tank with insulation. (d) It is found that the performance of the system with the evacuated tube collector is better, compared to other black paint flat plate collectors. (e) The minimum payback period is as 19 years for a system with underground storage tank while for the tank without insulation on ground is 34 years. (f) The saving of solar heating system with seasonal storage increases with the increase of the house load.
References [1] Nordell B, Hellstro¨m G. High temperature solar heated seasonal storage system for low temperature heating of buildings. Solar Energy 2000;69:511–23.
2539
[2] Breger SD, Hubbell JE, Hasnaoui HE, Sunderland JE. Thermal energy storage in the ground: comparative analysis of heat transfer modeling using u-tubes and boreholes. Solar Energy 1996;56:49–503. [3] Chung M, Park J, Yoon H. Simulation of a central solar heating system with seasonal storage in Korea. Solar Energy 1998;64:163–78. [4] Pahud D. Central solar heating plants with seasonal duct storage and shortterm water storage: design guidelines obtained by dynamic system simulations. Solar Energy 2000;69:495–509. [5] Reuss M, Beck M, Mu¨ller JP. Design of a seasonal thermal energy storage in the ground. Solar Energy 1997;59:247–57. [6] Argiriou AA. CSHPSS systems in Greece: test of simulation software and analysis of typical systems. Solar Energy 1997;60:159–70. [7] Ucar A, Inalli M. Thermal and economical analysis of a central solar heating system with underground seasonal storage in Turkey. Renewable Energy 2005;30:1005–19. [8] Ucar A, Inalli M. Exergoeconomic analysis and optimization of a solar assisted heating system for residential buildings. Build Environ 2006;41: 1551–6. [9] Duffie JA, Beckman WA. Solar engineering of thermal process. 2nd ed. New York: Wiley Interscience; 1991. ¨ nsal M, Dog˘antan ZS. Solar tables. Desing data for solar aided space heating [10] U system. Middle East Technical University, Gaziantep Campus; 1980.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal and economic comparisons of solar heating systems with seasonal storage used in building heating Aynur Ucar , Mustafa Inalli Department of Mechanical Engineering, Firat University, 23279 Elazıg˘, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 September 2007 Accepted 22 February 2008 Available online 9 April 2008
In this study, the thermal performances and economic savings of the three types of central solar heating system with seasonal storage are compared. Three types of seasonal storage were simulated: storage tank without insulation on ground, storage tank with insulation on ground, and underground storage tank without insulation. The long-term temperatures of water in the storage tank are calculated by finite element code ANSYSTM. The simulation results showed that the higher solar fraction and savings are obtained for system with storage buried into ground. Furthermore, the solar fraction of the storage tank system with insulation is significantly higher than that of without insulation storage system. Also, the solar fraction and savings of system with the evacuated tube collector are higher compared to other black paint flat plate collector. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Seasonal storage Building heating Solar energy
1. Introduction The rapidly increasing world population growth gives rise to a greatly increased demand for energy. To maintain the standard of living in industrialized countries and to improve the situation in developing countries, energy consumption must be much more efficiently utilized and energy supply includes a higher share from renewable sources. The store of solar heat from the summer to the winter for space heating is important because of large differences between solar energy supply and heat demand. Nordell and Hellstro¨m [1] evaluated the performance of a solar-heated low temperature space-heating system with seasonal storage in the ground using the simulation models TRNSYS and MINSUN together with the ground storage module DST. They developed an economically feasible design for a total annual heat demand of about 2500 MWh. It was found that total annual cost of the solar heating system was reduced by about 20% to about 125.6792$/MWh, which was lower than the best conventional alternative. Breger et al. [2] made a comparative analysis of the heat transfer from boreholes and U-tubes using analytical solutions, finite element modeling and the available simulation model. This analysis is used to support the development of a methodology by which the heat transfer of any U-tube configuration can be modeled by appropriately specifying parameters in the borehole storage simulation model. A central solar plant with
Abbreviations: TRNSYS, transient system simulation program; CSHPSS, central solar heating plant with seasonal storage; MINSUN, simulation program of central solar heating system. Corresponding author. Tel.: +90 0424 2370000, +90 0424 2375233. E-mail addresses: aucar@firat.edu.tr (A. Ucar), minalli@firat.edu.tr (M. Inalli). 0960-1481/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.02.019
seasonal storage (CSHPSS), which is under construction in Korea, is simulated using TRNSYS to predict thermal performances and economic aspects by Chung et al. [3]. Their system consists of two arrays of collectors, a medium-sized storage tank and two thermal loads. It was found that TRNSYS prediction for system is that about 39% of the total heating load of 885,000 MJ/year can be provided from the sun. Pahud [4] analyzed a central solar heating plant with seasonal ground storage by dynamic system simulations. He is investigated the thermal performance of the system consist of a collector area, water buffer storage and duct storage in the ground for different thermal loads. Reuss et al. [5] investigated the thermal performance of duct systems with vertical heat exchangers. Thermal performance of these systems is influenced by the heat and moisture movement in the area surrounding the heat exchangers. In this study, the combined heat and moisture transport was simulated on the computer for temperatures up to 90 1C. The design data of an interseasonal storage system in Greece are compared with the results of two CSHPSS simulation software codes, MINSUN and SOLCHIPS by Argiriou [6]. The comparison showed good agreement and therefore these userfriendly tools can be used with confidence for the design of CSHPSS under Greek weather conditions. In authors’ previous work [7], thermal performance and economic feasibility of two types of central solar heating system with underground seasonal storage under four climatically different Turkey locations were investigated. The effects of storage volume and collector area on the thermal performance and cost were studied for three load sizes. The temperature distribution in the ground was solved by using finite element method and finite element code ANSYSTM is chosen as a convenient tool. In the authors’ work [8], an exergoeconomic model was developed for
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
Nomenclature Ac Atpk CA CE CF df F I i MS N Np Qal
collector area, m2 ground contact area, m2 area dependent cost, $/m2 fixed cost, $ cost of energy from fuel, $/GJ down payment solar fraction monthly average daily solar radiation incident on the collector per unit area, MJ/m2 day assumed annual interest rate on mortgage solar system performance degrade period of economic analysis, year payback period of system, year design house heat load, W
analysis and optimization of solar heating systems with residential buildings. The optimum collector area (Ac) and storage volume (V) for solar assisted heating system in the Elazıg˘, Turkey (38.71N), by using MATLAB optimization toolbox were obtained. The energy and energy loss in each of the components of a solar heating system with seasonal storage were also determined. The most promising source of energy for Turkey is solar energy because of its climate. However, solar thermal is only used directly to a small extent. Therefore, the use of solar energy for residential building heating has become very important for the Turkish economy. For the economic design of the central solar heating system with seasonal storage, a theoretical study is necessary before installation. In this study, we compare the performance of the three types of central solar heating system with seasonal storage. The systems that can be simulated are: storage tank without insulation on ground, storage tank with insulation on ground and underground storage tank without insulation. The thermal performances of flat plate collectors and evacuated tube collectors are calculated in this study.
QB QS Qu Rv S (UA)B (UA)strg Ta Th Tcol Tiref Toref Ts W Z
heat load of building, W heat loss from the storage tank, W monthly average of useful solar energy, W resale value of solar system, $ solar savings, $ building loss coefficient, W/K storage tank loss coefficient, W/K ambient temperature, K temperature of fluid in heat exchanger, K collector temperature, K inside design air temperature, K winter design outside air temperature, K fluid temperature at the inlet to the collector, K heat pump power collector efficiency
where Ac is the collector area, I is the instantaneous solar radiation incident on the collector per unit area. Thermal performances of the black paint flat plate and vacuum tube collectors are researched in this study. The collector efficiency for the evacuated tube collector is given by Zeva ¼ 0:84 2:02ðT m T a Þ=I 0:0046I½ðT m T a Þ=I2
Zflat ¼ 0:70 3:4ðT m T a Þ=I
(3)
Efficiency curves for the evacuated tube and flat plate collectors are plotted in Fig. 2. It can be shown that the evacuated tube collector efficiencies are much better than those of the glazed flat plate collectors for the higher values of the temperature difference between absorber and ambient air.
A schematic diagram for the system consisting of plate solar collectors, a heat pump, a storage tank, and heat load is shown in Fig. 1. The heat produced by the collectors throughout the year is stored in the storage tank. This heat is used to provide house heating during the heating season. In the following paragraphs, modeling of the system components, including load and the method of solution are given. 2.1. Solar collectors The useful energy gain of collector surface area is given by the following equation [9]: (1)
(2)
where Tm is the mean collector temperature and Ta is the ambient air temperature. The efficiency for the flat plate collector is calculated by the following equation:
2. The thermal system
Q u ¼ ZAc I
2533
Fig. 2. Efficiency curves of vacuum tube and flat plate collectors.
Fig. 1. Schematic diagram of a solar heating system with storage tank on ground.
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2.2. Storage tank
2.3. Heating loads
In this study, a cylindrical tank is selected as a storage tank type and is an important part of a solar heating system. Selected glass wool is the insulation material of the tank which is placed on the ground. The heat loss from the tank on ground is calculated by
Solar energy is supplied from the storage tank to a building and instantaneous heat load for building is evaluated using the following equation:
Q s ¼ ðUAÞstrg ðT col T a Þ
A1 þ ðUAÞgnd ðk1 =d1 Þ þ ð1=hÞ
(5)
where k1 and d1 are the thermal conductivity and thickness of insulation and h is the convective heat transfer coefficient. In this study, the conduction resistance of the tank wall is neglected because the wall thickness is very thin. (UA)gnd represents heat loss through ground from the bottom of the tank and calculated using the following relation [3]: ðUAÞgnd ¼
Atpk ðd2 =k2 Þ þ ð4r=3prkgnd Þ
if d2 40:2r
k2 kgnd
(7)
The heat loss coefficient (UA)B of the building is calculated as
(4)
where (UA)strg is the loss coefficient of storage tank and calculated using the following equation [3]: ðUAÞstrg ¼
Q B ¼ ðUAÞB ðT iref T a Þ
ðUAÞB ¼ Q al =ðT iref T oref Þ
(8)
where Qal is the annual heating load of the building. If the energy demand of the building is provided by a heat pump, the instantaneous heat load of the building is Q B ¼ WðCOPÞ ¼ Wbp T h =½T h T s
(9)
where bp is the characteristic coefficient of the heat pump, which is in the range of 0.2–0.3. The solar fraction is defined as the utilized solar heat divided by the total heat demand and calculated from the following relationship: X F¼ ½Q u Q s =Q B (10)
(6) 3. Economic analysis
where k2 and d2 are the thermal conductivity and thickness of insulation on the tank bottom and kgnd is the thermal conductivity of the ground.
The economic analysis of solar heating system performed using the simplified P1 and P2 method [9]. Solar savings S, are given by S ¼ P1 C F Q B F P2 C S
Table 1 Simulation parameters of the storage tank and load
(11)
where CS is calculated as follows: C S ¼ C A Ac þ C E Parameters
Storage tank Type Insulation material Insulation thickness Insulation thermal conductivity
Storage tank on ground Glass wool 0.5 m 0.026 W/m K
Load Design house heat load, Qal Building loss coefficient, (UA)B Inside design air temperature, Tiref Winter design outside air temperature, Toref
315360 MJ/year (per house) 9556 MJ/year K (per house) 294 K 261 K
(12)
where CA is the total area dependent co‘ts for the solar system ($/m2). These costs include the cost of the collector and part of the cost of the thermal storage equipment. CE is the total cost ($) of the equipment, which is independent of the collector area. Examples are costs for piping or ducts, controls, blowers, and so on. P1 and P2 are calculated by P1 ¼ ½ð1 þ df Þ=ðdf iÞ½1 ðð1 þ iÞ=ð1 þ df ÞÞN
(13)
P2 ¼ 1 þ P 1 M S Rv ð1 þ df ÞN
(14)
The payback period Np is Np ¼ ln½1 ðP 2 C S =C F Q B FÞðdf iÞ=ð1 þ df Þ= ln½ð1 þ iÞ=ð1 þ df Þ
Table 2 Economic parameters
(15)
Type
Collector types
Area dependent cost, CA (per house) Fixed cost, CE (per house) Cost of energy from fuel, CF Solar system performance degrade, MS Down payment, df Assumed annual interest rate on mortgage, I Resale value of solar system, Rv (per house) Period of economic analysis, N
4. Simulation parameters
Flat plate
Evacuated tube
1200$/m2 4000$ 50$/GJ 1% 5% 5% 4000$ 50 years
1628$/m2 4430$ 50$/GJ 1% 5% 5% 4430$ 50 years
In this study, water temperature in storage tank of the solar system is calculated by finite element code ANSYSTM. The monthly outside air temperature and net energy input rate in the storage tank were transferred to ANSYS as the input data and monthly average water temperature in the storage is found for the periodic operation regime. The solar systems that can be simulated are flat plate collectors and evacuated tube collectors. The load size considered was 25,
Table 3 Monthly weather data for Elazıg˘ [10]
Ta (1C) I (MJ/m2/day)
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jun.
27.2 18.3
27.0 19.2
22.0 17.5
14.8 13.5
7.8 8.5
1.5 6.9
1.3 7.1
0.0 10.7
4.7 12.6
11.8 15.4
17.4 15.6
22.9 18.1
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
250, and 1000 house units. The systems that can be simulated are: storage tank on ground (without insulation), storage tank on ground (with insulation), and underground storage tank (without
Average temperature (°C)
120
Underground storage tank (without insulation) Storage tank on ground (with insulation) Storage tank on ground (without insulation)
100 80 60
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insulation). The detailed data required for simulation of these three seasonal storage types and heating load are given in Table 1. The economic analysis of solar heating system performed by using the simplified P1 and P2 method and parameter values used in the economic analysis are given in Table 2. Simulations were performed for Elazıg˘ location in Turkey, which is at latitudes between 36–421N. The weather data used in the simulation are average values based on many years of measurement in meteorological stations in Turkey. The data for the monthly average solar radiation on a horizontal surface and the monthly average outside air temperature of this location are given in Table 3.
40
5. Results and discussion
20 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 Time (year)
Fig. 3. Temperature variation water in storage tank during a simulation.
The results in this study are valid for a periodic operation case of the system. Fig. 3 shows water temperature variation in storage tank over years. In order to reach the periodic operation regime, the system with underground storage tank requires approximately 23 years. When a storage tank without insulation on
Fig. 4. Solar fraction as a function solar collector area for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
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ground is used, required time to reach periodic operation regime of the system is 5 years. The maximum storage tank temperature for this system is set to 40 1C, while it is 100 1C for the system with underground storage tank. It can be seen that water temperature difference between the systems with insulation and without insulation on ground is 50 1C. The solar fraction as a function of solar collector area for three loads calculated using the weather data for Elazıg˘ are given in Fig. 4. The solar fraction increases with increase of solar collector area for three seasonal storage types. The heat loss from the storage in underground storage system is less than that of the system with storage tank on ground and therefore a smaller collector area is required in order to achieve higher solar fraction value. It is observed that when storage tank with insulation on ground is used, the collector area required in order to achieve 0.70 solar fraction is 22 m2/house, i.e. 9% bigger than that of the underground storage tank system. Fig. 5 shows the solar fraction as a function of storage volume for three loads. The solar fraction increases with increasing storage volume. The higher solar fraction takes place for the
system with underground storage tank, compared with the other two storage systems. Using same storage volume (80 m3/house), solar fraction value is reached to 0.72 for the storage tank with insulated on ground and it is 0.60 for storage system without insulation on ground. The solar saving as a function of solar collector area for three loads is given in Fig. 6. The solar saving of system increases with increase of solar collector area for three types of storage. It can be seen that saving of the system with underground storage tank is higher than that of the other two storage systems. In the case of the 250 house loads and 20 m2/house collector area, saving of underground storage tank system is 15% higher than that of storage tank with insulation on ground and 47% than of noninsulated storage tank on ground. Fig. 7 shows the solar saving as a function of storage volume for three loads. The solar fraction increases with increasing storage volume and therefore the savings of solar system increases depends on storage volume. The higher solar saving takes place for the system with underground storage tank, compared with the other two storage systems.
Fig. 5. Solar fraction as a function storage volume for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
Savings ($)
2.0E+07
2.5E+08
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
2.0E+08 Savings ($)
2.4E+07
1.6E+07 1.2E+07 8.0E+06
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1.5E+08 1.0E+08 5.0E+07
4.0E+06
0.0E+00
0.0E+00 10
15
20
25
10
30
15
1.0E+09
Savings ($)
8.0E+08
20
25
30
Collector area (m2/house)
Collector area (m2/house) Storage tank on ground (without insulation) Storage tank on ground (without insulation) Storage tank on ground (without insulation)
6.0E+08 4.0E+08 2.0E+08 0.0E+00 10
15
20
25
30
Collector area (m2/house) Fig. 6. Solar saving as a function solar collector area for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
2.5E+07
1.5E+07
2.5E+08
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
2.0E+08 Savings ($)
Savings ($)
2.0E+07
1.0E+07 5.0E+06
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
1.5E+08 1.0E+08 5.0E+07
0.0E+00
0.0E+00 40
60
80
100
120
40
Storage volume (m3/house) 1.1E+09
Savings ($)
9.0E+08
60
80
100
120
Storage volume (m3/house)
Storage tank on ground (without insulation) Storage tank on ground (with insulation) Underground storage tank
7.0E+08 5.0E+08 3.0E+08 1.0E+08 40
60
80
100
120
Storage volume (m3/house)
Fig. 7. Solar saving as a function of storage volume for three load sizes: (a) 25 houses, (b) 250 houses, and (c) 1000 houses.
The effect of the load size upon the solar saving is illustrated in Fig. 8. This figure shows that the saving increases with the increase of the house load. It can be seen that the saving for 1000 house loads is 63% higher than that of 500 house loads. Fig. 9 shows effect of collector efficiency on solar fraction and savings for the three load sizes. The solar fraction and savings of systems with the evacuated tube collector which has an evacuated absorber strip are higher, compared to other black paint flat plate collectors. It is observed that the required surface area for the flat plate collectors is larger than that of evacuated tube collectors. For example, the required collector area to achieve a solar fraction of 70% for 25 house loads is 18 m2 for evacuated tube collector and 20 m2 for the flat plate collector. The payback period and solar fraction for three locations are given in Table 4. The payback period was evaluated
by using a system lifetime of 50 years. These results show that the payback period of system for the tank with insulation on ground is 10% higher than that of a system with underground storage tank. The minimum payback period of system with underground storage tank is obtained as 19 years. For 20 m2 collector area, the payback period of system for the tank without insulation on ground is 42 years, while for underground storage tank it is 23 years. The performance of a solar heating system with seasonal storage is directly related to the water temperature of the storage tank. The effect of ambient temperature on the water temperature in storage for system with underground storage tank is less than of the other two storage systems. Therefore, the lowest payback period occurs with the system with underground storage tank.
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6. Conclusions
solar collector efficiency on performance of system is discussed. The following items can be summarized from these analyses:
In the present study, thermal performance and economic feasibility of the three types of central solar heating system with seasonal storage in Turkey is investigated. In addition, the effect of
(a) The higher solar fraction is obtained for system with storage buried inside ground for three load sizes. The solar fraction increases with increase of solar collector area for three seasonal storage types. (b) It is shown that the solar saving of the system with underground storage tank is higher than that of the other two storage systems. The heat loss from a tank without insulation which is placed on the ground is higher than that of tank with insulation and therefore savings of this system are much higher.
Table 4 Payback period and solar fraction for the three systems (25 houses, V ¼ 40 m3/ house)
Fig. 8. Solar savings of the storage tank system with insulation on ground for three load sizes.
Collector Storage tank on ground (without insulation) area, A (m2/ Payback Solar house) fraction, period, N (years) F
Storage tank on ground Underground storage (with insulation) tank Payback Solar fraction, period, N (years) F
Payback Solar fraction, period, N (years) F
10 15 20 25 30
0.30 0.46 0.61 0.76 0.90
0.32 0.49 0.67 0.81 0.97
0.18 0.27 0.37 0.47 0.57
34 40 42 43 43
Fig. 9. Effect of collector efficiency on solar fraction and savings for the three loads in Elazıg˘.
21 24 25 26 27
19 22 23 25 25
ARTICLE IN PRESS A. Ucar, M. Inalli / Renewable Energy 33 (2008) 2532–2539
(c) The required storage volume to achieve a certain solar fraction for storage tank with without insulation on ground is bigger than that of storage tank with insulation. (d) It is found that the performance of the system with the evacuated tube collector is better, compared to other black paint flat plate collectors. (e) The minimum payback period is as 19 years for a system with underground storage tank while for the tank without insulation on ground is 34 years. (f) The saving of solar heating system with seasonal storage increases with the increase of the house load.
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