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December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman This paper has been presented at the TRNSYS user day, March 26, 2004, Stuttgart, Germany Page 1

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Page 1: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in

TRNSYS

M.J.M. Jong M.J. Elswijk

K.J. Strootman

This paper has been presented at the TRNSYS user day, March 26, 2004, Stuttgart, Germany

Page 1

Page 2: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Simulating PV-Thermal collectors in large solar systems in TRNSYS

M. J.M. Jong, M.J. Elswijk, K.J. Strootman

Energy research Centre of the Netherlands ECN Unit Renewable Energy in the Built Environment

P.O. Box 1, 1755 ZG Petten, The Netherlands Tel +31-224-564938, Fax: +31-224-568966

E-mail: [email protected] Abstract PVT panels generate electric power and heat simultaneously. PVT panels can be used par excellence on places where there is a lack of available roof area, for example on high-rise buildings. A study1 [1] of 3 high-rise buildings in the Netherlands provided with a solar system for space heating and domestic hot water shows that with PVT panels the annual saved prime energy quantity will be 0.45 GJ/m2 more than in the case with solar collectors only. A separate system of solar collectors and PV requires 38 % more roof area than a PVT system with the same amount of annual electricity and heat production. The whole system is simulated with standard types (and equations) in TRNSYS 15. The simulation of the control system requires a lot of work. Shading of the PVT collectors itself (because they are mounted in rows behind each other) has an influence on the annual energy production. The influence of shading on the electric energy production is bigger than on the thermal energy production. The shading effects of the PVT collector fields are calculated with equations in TRNSYS. There is no special type available and this would be very helpful. One of the objectives of this study was to find a solar collector area with a determined amount of energy production. In this study, this was done by simulating the system a lot of times with different solar collector areas just as long as the energy production complies with that amount of energy. A kind of optimisation toolbox would be helpful for this, but so far this is not included in TRNSYS. Introduction A photovoltaic/thermal panel, or PVT panel, is a combination of photovoltaic cells with a solar thermal collector, forming one device that converts solar radiation into electricity and heat simultaneously. The excess heat that is generated in the PV cells is removed and converted into useful thermal energy. As a result, PVT panels generate more solar energy per unit surface area than a combination of separate PhotoVoltaic (PV) panels and solar thermal collectors. Moreover, PVT panels share the aesthetic advantage of PV.

Figure 1: Front and back of a PVT panel 1 This work has been realised in part by support of the Renewable Energy in the Netherlands research program, implemented by the Dutch Organisation for Energy and Environment Novem, in commission of the Ministry of Economic Affairs.

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Page 3: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

PVT-collectors (PVT panels in a collector box) can be used on places where there is limited area available. If separate systems for PV and solar collectors will be used, relatively a lot of area is necessary to produce the same amount of electricity and heat. If the total available area is insufficient, PVT-collectors can be an alternative. On high-rise domestic buildings, the available roof surface per dwelling is relatively small compared to that of low-rise buildings. This means that per dwelling less roof area is available for harvesting solar energy. Available solar technologies for high-rise buildings are for example PV-modules and solar thermal collectors. Because of the limited roof area of high-rise buildings, PVT-collectors can be used. ECN examined three high-rise buildings with solar thermal systems in The Netherlands to determine the energy-related and technical differences for the application of PVT-collectors in large solar thermal systems. As a reference, systems with solar thermal collectors only and systems with a combination of PV-panels and conventional solar thermal collectors are defined. TRNSYS is used to compare the different systems. The selected cases are real buildings for which (conventional) solar collectors have already been planned. The solar systems provide houses with hot water and space heating. The numbers of houses in the building for the selected cases are 32, 61 and 78. The majority of the houses are flats, but there are also a few single-family dwellings and penthouses. To determine whether PVT panels can be applied in a system on high-rise buildings, the cost price is important. The cost price of a PVT panel is difficult to estimate because the number of PVT panels that will be produced (mass production or not) influences the cost price. Nevertheless the cost price is taken into account in this study. In this paper however this is left out of consideration. Description of the solar thermal system The principle of the solar systems for the 3 cases is the same. A diagram of the system as it will be built is shown in Figure 2.

Figure 2: Diagram of solar thermal system

The solar system consists of: • Solar collectors • Storage tank for solar collector • Storage tank for hot water • Heat exchangers • Gas absorption heat pump • Auxiliary heaters • Ring lines central heating and hot water

Page 3

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Welk “work”? Een korte intro geven van het betreffende project …
Page 4: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Solar collectors The solar collectors transform sunlight into heat and the heat will be stored in the storage tank for the solar collector. Depending on the temperature, the hot water from the solar collectors can enter the storage tank on different levels. Storage tank for solar collector The stored heat in the storage tank can be used in three ways: • Directly for space heating • For the hot water storage tank • As source for the gas absorption heat pump. If the temperature of the water in the storage tank for the solar collector is 4 °C higher than the temperature of the return water of the central heating system and there is a heat demand, there will be heat transferred with a heat exchanger from the storage tank to the water of the central heating system. The water in the storage tank for the solar collector will be used for domestic hot water if the temperature of the water in this storage tank is 4 °C higher than the temperature of the water in the storage tank for hot water. If the temperature of the water in the storage tank for hot water reaches 68 °C or if the temperature difference is reduced to 2 °C, the heat transfer stops. The water in the storage tank for the solar collector will be used as source for the gas absorption heat pump if the temperature of the water is between 6 and 22 °C. The water can be extracted from different levels of the storage tank. Storage tank for domestic hot water This storage tank provides the houses with hot water. The storage tank is connected with a ring line and each house uses water from this storage tank. This storage tank has an internal heat exchanger for exchanging heat from the storage tank for the solar collector to this storage tank. If the temperature of the water at the bottom of the tank is more than 150 hours below 45 ° C, water from the bottom of the tank will be heated by one of the auxiliary heaters. This way, possible legionella problems will be avoided. At the bottom of the tank, there is a cold water supply. Heat exchangers The system consists of 2 heat exchangers: one for exchanging heat from the storage tank for the solar collector to the water for the central heating system and one for exchanging heat from the auxiliary heaters to the storage tank for hot water. Gas absorption heat pump The gas absorption heat pump uses natural gas. The evaporator of the heat pump is connected with water of the storage tank for the solar collector. The condenser of the heat pump is connected with the ring line for space heating. Using the heat pump will cool the water in the storage tank for the solar collector and heat the water in the ring line for space heating. Auxiliary heaters The auxiliary heaters will be used for space heating and for heating hot water, including preventing legionella problems. The auxiliary heaters use natural gas.

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Page 5: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

The auxiliary heaters heat the water for space heating to a temperature that depends on the ambient temperature (heating curve). The water in the storage tank for hot water will be heated to a temperature of 65 °C. Ring lines for central heating and hot water The solar system has 2 ring lines: one for space heating and one for hot water. Water in the ring line for space heating only runs when there is a heating demand for space heating. Water in the ring line for hot water runs 8760 hours a year. Both ring lines are both aboveground and underground. Both ring lines are completely insulated. Modelling the system in TRNSYS Solar collectors, PVT panels and PV For the solar collectors, the standard solar collector model of TRNSYS (type 1) is used. The used collectors for the 3 cases are tested by TNO (Dutch research institute) to determine the intercept efficiency and the first-order-coefficient of the efficiency [2]. In the case of PVT-panels (one of the variants that are simulated), type 1 is also used to simulate the thermal energy production of the panel. The PVT-panels are tested at ECN [3]. In Table 1, the parameters of the solar collectors and the PVT panels are shown.

Parameter Solar collector PVT panel intercept efficiency 0.76 0.60 first order coefficient 3.47 m2 /W K 5.00 m2 /W K

Table 1: Parameters for solar collector and PVT collector

The electrical efficiency (ηel) of a PVT collector is calculated as shown in the formula below [4].

)25(00045.0092.0 −−= PVel Tη

Tpv is the temperature of the PV cel. For calculating Tpv two different cases are distinguished: The situation that there is water running through the PVT collector and the situation that there no water running through the collector. In the first situation, Tpv is the average of the fluid inlet and outlet temperature. For the second situation, the formula as shown below is used [5].

totampv ITT1000

40+=

With: T pv : Temperature of PV cell [°C] T am : Ambient temperature [°C] I tot : Total solar irradiation on PV cell [W/m2] Also, a case of separated solar collectors and PV panels is simulated. In this case, for the electrical efficiency of the PV panels, the formula below is used. This efficiency is 8% higher than the electrical efficiency of a PVT collector. The additional glass layer of a PVT collector compared with a PV panel causes the difference of 8 % in the efficiency.

)25(00045.0100.0 −−= PVel Tη The formulas are entered as equations in TRNSYS.

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Page 6: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Storage tanks For modelling both storage tanks, type 4 of TRNSYS is used. Because the flow can enter and leave the storage tank for the solar collector on 4 different levels (depending on the temperature), the model of the storage tank is used 4 times. The flows between the storage tanks are calculated with equations. Gas absorption heat pump Not many data is known about the gas absorption heat pump. This is the reason that a simple model is used to simulate the thermal behaviour of the heat pump. The thermal energy output is independent of the temperature of the water that enters the evaporator. The gas absorption heat pump is simulated with the help of type 6 (auxiliary heater) in TRNSYS. The heating power (condenser) is 36.6 kW and the cooling power (evaporator) is 17.1 kW. The gas absorption heat pump is provided with a flue gas condenser with which 3.26 kW heat will be recovered. Heating demand houses The heating demand of the houses (flats, single-family dwellings and penthouses) is not simulated simultaneously with the solar system. The simulation of the solar system uses heating demand data from a file. The file contains hourly heating demand data of a flat and a single-family dwelling (the heating demand of the penthouse is supposed to be equal to the heating demand of the single-family dwelling). The file with heating demand data is a result of a TRNSYS simulation of the flat and the single-family dwelling. Including the simulation of the heating demand of the flats and the single-family dwellings in the simulation of the solar system would make the simulation needless slow and for this reason this is avoided. The insulation values of the walls and the roofs of the houses are 2.7 and 3.2 m2 K/W respectively. The flats and the houses are provided with windows with an U-value of 1.7 W/m2 K. All houses and flats are provided with a ventilation system with heat recovery. The thermal efficiency of the heat recovery is 95 %. The hourly heating demand (cumulative) of the flat and the single-family dwelling are shown in Figure 3.

Heating demand flat and single-family dwelling as function of time

0

2

4

6

8

10

12

14

16

0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760

Time [hours]

Hea

ting

dem

and

[GJ]

Heating demand flat

Heating demand single-family dwelling

Figure 3: Heating demand of flat and single-family dwelling

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Page 7: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Hot water pattern The average number of people that live in a flat and a single-family dwelling is assumed to be 1.5 and 3 respectively. According to a Dutch standard [6] the daily quantity of hot water (60 °C) for 1.5 and 3 persons is 52.2 and 105 litres. For these simulations, this quantity is assumed to be spread over the day

as shown in Figure 4.

igure 4: Domestic hot water pattern for flat and house

is supposed that in the morning and in the evening the inhabitants of the houses will take a shower

in

eat losses in ring lines

he heat losses (Φ) of a pipe can be calculated with

Hot water pattern as function of time for different houses

0

1

2

3

4

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour of the day)

War

m w

ater

flow

[litr

e/qu

arte

r of a

n ho

ur]

Hot water pattern flat

Hot water pattern sinlge-family dwelling

F Itand this causes the 2 peaks in the pattern (1.5 and 3 litres per quarter of an hour). All other use of hot water is supposed to be constant during the day (0.375 and 0.75 litres per quarter of an hour). The relatively big number of houses that is connected to 1 hot water system justifies the constant levelsthe hot water pattern. H T

∑++

−=Φ

λαα

πiu

uuii

rrrr

TTlln11)(2 21

ith:

Length of the pipe [m] let of the pipe [°C]

]

t [W/m2 K]

the pipe (or insulation) material [W/m K]

the simulation, each ring line is split into 3 pieces. One piece of the ring line is situated in a technical room (aboveground), another part is situated underground and the third part is aboveground but after

W l:T1: Water temperature at the inT2: Ambient temperature of the pipe [°C] αi: Inside heat transfer coefficient [W/m2 Kri: Internal radius of the pipe [m] αu: Outside heat transfer coefficienru: External radius of the pipe [m] λ: Heat conductivity coefficient of In

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Page 8: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

the use of hot water (hot water ring line) or transferred heat (ring line central heating system). The 3 calculated heat losses of each ring line are the total heat losses of the whole ring line. Temperature T1 is not constant in a one piece of the ring line. Due to the heat loss of the water in the ipe, this temperature will decrease. To calculate the heat losses correctly, a piece has to be split into

t

el, but it is also possible to use type 1 (pipe or duct) of TRNSYS.

rovided with 10 circulation pumps. The power of these pumps varies from 5 to 450 W. Counting the running hours in the model and multiply this with the power simulates the

the three high rise buildings, the solar collectors are in one row and therefore solar collectors o not shade other solar collectors. On another building, the collector arrays are located staggered. This

lar collectors are located in 4 rows on the roof. The angle of the solar ollectors with the flat roof is 17 °. The angle is not ideal, but because of aesthetic aspects, this angle

olar

uses shading for zeniths < 24 ° and < 31 ° (the distance between the ollector fields is not equal). For every time step of the simulation, the shaded area on 3 fields in the

d.

he

llectors, the PVT collector fields are configured in a different way an the solar collectors. This is because shading effects have a bigger influence on the electrical

pseveral pieces. The temperature decrease is only a few tenth degrees Celsius, so for calculating the healosses a constant value of T1 for the whole ring line can be used. The ring line heat losses are calculated with equations in the mod3 Power circulation pumps The whole solar system is p4energy consumption of each pump. Shading On one ofdreduces the shading effect. On the third building, the soccannot be bigger. The collector fields are located behind each other and this causes shading on the scollector fields in the back. The geometry of collector fields cacback is calculated. This way a total fraction of shaded area can be calculated. With this calculated fraction, the direct radiation on the solar collector can be calculated. The position in arrays causes alsoa reduction in the view factor for diffuse radiation. This view factor is constant and can be calculateThe value of the view factors is 0.81 and 0.85. With these values, the diffuse radiation for every collector field can be calculated. With the shading factor for direct radiation, a new direct radiation can be calculated, taking the shaded part into account. Summing the diffuse and direct radiation gives ttotal radiation. Because of the small angle (17 °) of the collector fields with the roof (horizontal), ground reflectance is neglected. In the case of simulating PVT cothefficiency than on the thermal efficiency. If a PV field is 25 % shaded, the whole panel produces no electricity anymore while a solar collector still produces heat. According to [7] the PV must be totallyunshaded for zeniths > 12 °. This can be realised by increasing the distance between the PVT collector fields. This implies that for an equal area of PVT collectors (if it is compared to solar collectors) more roof area is necessary.

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Page 9: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Results of the simulation In total, 3 cases are simulated. In this paper, only the results of one case will be mentioned to avoid a big number of numerical results. The relevant parameters for the simulation of this case are shown in Table 2. Number of houses [-] 78 Solar collector area [m2] 184 Volume storage tank for solar collector [litre] 6000 Thickness of insulation storage tank for solar collector [cm] 10 and 15Volume storage tank for hot water [litre] 1400 Thickness of insulation storage tank for hot water [cm] 10 Length of ring line space heating system [m] 315 Length of ring line hot water system [m] 315

Table 2: Parameters simulation solar system

For each high-rise building several simulations are executed. The first one is the one, as the solar system will be built (with solar collectors). The second is the one with the same collector area, but with PVT collectors instead of solar collectors. The third simulation is the one with solar collectors and (separate) PV with an equal heat and electricity production as in the situation with PVT collectors. The fourth is the one with PVT collectors with the same heat production as in the situation with solar collectors. An overview of the simulations is shown in Table 3. Run Configuration Heat and electricity production

1 A m2 solar collectors Q1 GJ/year (thermal) no electricity 2 A m2 PVT collectors Q2 GJ/year (thermal) and E1 kWh/year (electrical) 3 B m2 solar collectors and C m2 PV Q2 GJ/year (thermal) and E1 kWh/year (electrical) 4 D m2 PVT collectors Q1 GJ/year (thermal) and E2 kWh/year (electrical)

Table 3: Overview of simulations

Runs 1 and 2 are the result of one simulation, but for run 3 and 4 several simulations are necessary. The heat and electricity production are results of the simulation, depending on the PVT collector or solar collector area. This has to be varied several times to get this production equal to the ones from previous simulations. In Table 4, the different areas and other parameters for the different runs are shown. Run 1 Run 2 Run 3 Run 4 Collector area 184 n/a 98 n/a PVT area [m2] n/a 184 n/a 234 PV area [m2] n/a n/a 161 n/a Angle solar collec-tors/PVT/PV [degrees] 17 17 17 37 Volume storage tank solar collector [litre] 6000 6000 6000 18000

Table 4: Collector/PV/PVT areas for the different runs

To avoid a very large PVT area for run 4, the angle of the solar collectors with the horizontal and the volume of the storage tank for the solar collectors are optimised to get better results. Results run 1 (184 m2 solar collector) This run gives results of the system as it will be built in reality. The solar contribution to the heating demand for space heating and for hot water is shown in Figure 5 and Figure 6.

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Page 10: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Overview energy quantities heating demand space heating (totally app. 709 GJ)

2%9%

89%

Auxiliary heatersSolar collectorsHeat pump

Figure 5: Pie chart of energy quantities for space heating

igure 6: Pie chart of energy quantities for hot water

Table 5 all numerical results are shown for the 4 variants of one case.

Overview energy quantities heating demand hot water (totally app. 564 GJ)

70%

30%

Auxiliary heatersSolar collectors

F

In

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Page 11: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Energy quantity

184 m2 solar collector

184 m2 PVT

90 m2 solar

collector + 161 m2 PV

234 m2 PVT

Heat demand hot water [GJ/year] 356.6 356.6 356.6 356.6 Heat demand space heating [GJ/year] 759.1 759.1 759.1 759.1 Heat production heat pump [GJ/year] 69.5 69.2 61.3 128.3 Heat production auxiliary heaters space heating [GJ/year] 691.6 703.0 712.0 632.9 Heat production collectors [GJ/year] 221.8 163.7 163.2 228.2 Heat production auxiliary heaters hot water [GJ/year] 394.4 438.3 434.5 418.0 Heat losses storage tank for solar collector [GJ/year] 7.9 5.2 5.4 13.1 Heat losses ring line for space heating [GJ/year] 23.5 22.1 22.1 22.4 Heat direct used for space heating [GJ/year] 17.0 6.8 5.9 17.5 Heat production auxiliary heaters legionella [GJ/year] 2.7 3.2 3.3 3.4 Heat production solar collector for hot water [GJ/year] 166.5 121.9 125.6 142.2 Heat losses ring line for hot water [GJ/year] 203.0 203.1 203.1 203.1 Heat losses storage tank for hot water [GJ/year] 4.5 4.2 4.3 4.4 Electricity production [GJ/year] 0.0 55.1 55.0 72.0 Electricity use for circulation pumps [GJ/year] 15.8 15.3 13.3 17.1

Table 5: Numerical results simulation solar systems and variants

The heating demand for space heating and the heating demand for domestic hot water include the losses for the pipelines. Especially the ring line losses for hot water are very high. This is caused by the number of hours that there will be water running through the ring line (8760 hours a year) and the relatively high temperature of the water (65 °C). The solar collectors produce most of the time heat that will be used for hot water. This is quite logical. To use the heat for space heating, the temperature must be high enough for exchanging heat. The temperature of the water in the storage tank solar collectors is high in summertime and this is the time that there is no heating demand for space heating at all. Because there is always heating demand for hot water, all heat that will be produced in summertime will be used for hot water. One of the aims of this study is to determine if PVT panels can be an alternative for harvesting as much solar energy as possible on high-rise buildings. To compare different energy quantities is difficult because some energy quantities will be generated with different efficiencies. For example the thermal efficiency of the auxiliary heater for generating heat for space heating is different than for generating heat for hot water. Especially when electricity is generated (PVT collectors or PV panels) there must be a way to compare electricity with heat. To make a good equation between the different energy flows (heat and electricity) all energy quantities can be expressed in prime energy. Prime energy is the base material with which energy can be made (for example natural gas and coal). To convert energy quantities into prime energy, the efficiencies must be known. In this case, 3 efficiencies are important. These assumed efficiencies are shown in Table 6. The efficiency for electricity is an average of the power supply in the Netherlands.

Efficiency Value Auxiliary heater for space heating 97 % Auxiliary heater for hot water 90 % Electricity production 39 %

Table 6: Efficiencies for calculating prime energy

With these efficiencies it is possible to calculate all energy flows in prime energy. The result is shown in Table 7.

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Page 12: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

Energy quantities

184 m2 solar

collector

184 m2 PVT

90 m2 solar

collector + 161 m2 PV

234 m2 PVT

Space heating demand prime energy [GJ/year] 806.8 805.3 805.4 805.7 Hot water heating demand prime energy [GJ/year] 626.8 626.6 626.6 626.7 Total heating demand prime energy [GJ/year] 1433.6 1431.9 1432.0 1432.4 Auxiliary heater space heating prime energy [GJ/year] 713.0 724.8 734.0 652.5 Auxiliary heater hot water prime energy [GJ/year] 441.2 490.6 486.5 468.1 Prime energy gas absorption heat pump [GJ/year] 39.7 39.5 35.0 73.2 Total prime energy demand space heating and hot water [GJ/year] 1193.9 1254.8 1255.5 1193.8 Saved prime energy by using solar collectors/PVT collectors [GJ/year] 239.7 177.1 176.5 238.5 Saved prime energy by electricity production PV [GJ/year] 0.0 141.2 141.1 184.6 Total saved prime energy [GJ/year] 239.7 318.3 317.6 423.1 Required area (PVT, solar collector, PV) [m2] 184 184 259 234 Saved prime energy [GJ/m2 year] 1.30 1.73 1.23 1.81

Table 7: Energy quantities in prime energy

As mentioned before, all simulations are executed for 3 cases. Every case is simulated 4 times (see Table 4). Every case has a different number of houses and different areas of solar collectors, PVT collectors and PV. In Table 8, the results of the saved prime energy per m2 solar system (Solar collector, PVT collector or Solar collector and PV) are shown for the 3 cases. Variant Run 1 Run 2 Run 3 Run 4 Case 1: Saved prime energy [GJ/m2 year] 1.30 1.73 1.23 1.81 Case 2: Saved prime energy [GJ/m2 year] 1.08 1.57 1.15 1.62 Case 3: Saved prime energy [GJ/m2 year] 1.13 1.58 1.15 1.78 Average: Saved prime energy [GJ/m2 year] 1.17 1.62 1.18 1.74

Table 8: Saved prime energy per m2 solar system for 3 cases

Especially for high-rise buildings, the roof area is limited. It could be possible that a solar system that consists of solar collectors and PV panels (run 3) or of a larger PVT area (run 4) will not fit on the roof. In Table 9 the required and the available roof area is shown for each run and for each case. The shading effect is taken into account for calculating the available roof area. Case Area Case 1 Area Case 2 Area Case 3 Desired Available Desired Available Desired Available Run number m2 m2 m2 m2 m2 m2 Run 1 184 184 136 136 163 164 Run 2 184 184 136 115 163 163 Run 3 259 190 185 139 222 139 Run 4 234 138 171 77 192 not an option

Table 9: Desired and available roof area for the different cases

For case 1 there is no shading effect because the collector fields are located in 1 row. The shading effect of case 3 is neglected because the solar collector fields are located staggered. In case 2 the shading effect is relevant. To avoid a big influence of shading for PVT collectors, it is sometimes necessary to enlarge the distance between the rows of the collector fields. This is the reason why in

Page 12

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Available is een betere term, zie ook verderop.
Page 13: PVT-collectors in large solar thermal systems · December 2004 ECN-RX--04-028 Simulating PV-Thermal collectors in large solar systems in TRNSYS M.J.M. Jong M.J. Elswijk K.J. Strootman

case 2 it is not possible to have the same area PVT collectors as solar collectors. As shown in Table 9, with solar collectors an area of 136 m2 is possible and with PVT collectors the maximum available area is 115 m2. In some other cases there is not enough roof area available for the different options. These cases are printed bold in Table 9. The influence that shading has on the thermal and electrical energy production of PVT collectors is also simulated. Two simulations are executed for case 2 (one with 136 m2 PVT collectors without shade and one simulation with 136 m2 PVT collectors with shade (shading starts at zeniths < 12 °). The results are shown Table 10. Without shading With shading Thermal energy production [GJ/year] 100.9 98.3 Electric energy production [GJ/year] 40.1 38.8

Table 10: Influence of shading on electricity and heat production PVT collector

Conclusion By using PVT collectors instead of solar collectors, the saved annual prime energy (for 3 cases in this study) is 0.45 GJ per m2 higher. Although the thermal energy production reduces by using PVT panels instead of solar collectors, this loss is compensated by the electricity production of the PVT collector. The same heat and electricity production can be achieved by installing solar collectors and PV panels separately. This requires 38 % more area. In none of the cases there is enough roof area to install a separate system for solar collectors and PV. To enlarge the PVT area to get the same thermal results as in the situation with solar collectors is also not possible. Sometimes it is not possible to replace the solar collector area by an equal area of PVT collectors. This is caused by the fact that PVT collectors can have another configuration and require sometimes a larger distance between the fields because shading has a bigger influence on the electric efficiency of PVT collectors than on the thermal efficiency of solar collectors. Of all the studied systems in this project, the application of PVT collectors saves the most prime energy per square meter. For high-rise buildings this is a real option to save as much energy as possible. The other options require more roof area for the same energy production and for high-rise buildings this is often not possible because the roof area is limited. Shading effects are rather important to take into account with simulating a solar system, especially for PVT collectors. Because shading has a big influence on the electricity production of PV, this has to be prevented as much as possible, also for PVT collectors. If the PVT collectors are located in rows behind each other, the distance between the rows must be that big that a row starts shading another row for zeniths < 12 °. This implies that for an equal amount of PVT collector area (compared with collector area) more roof area is available and sometimes this is not possible. If the PVT collectors are placed in such a way that shading starts at zeniths < 12 °, the thermal and electric energy production are 2.6 and 3.2 % less than in the situation that there is no shading at all (PVT collectors are located in one row). With TRNSYS it is possible to simulate solar thermal systems in a detailed way. A lot of standard components can be used. Unfortunately there is no standard model available for a storage tank with multiple in- and outputs. To simulate this, the standard storage tank has to be used several times and this requires a lot of equations to calculate the flows between the different tanks in the model. The relative big water flows compared to the volume of the storage tank require a small time step. Due to the complexity of the system in combination with the small time step, the runtime is rather long (app. 12 hours on a 933 Mhz Pentium). By using the parametrics table, simulations can be done successively automatically. The aim of run 3 was to find a certain collector area for a determined amount of heat production. In this case this was done by executing a simulation with a certain collector area. The simulated heat production was compared with the determined heat production and with these data a new collector area

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was calculated and with this area a new simulation was executed. It took several runs to find an answer. A kind of optimisation toolbox would be very helpful for this, but this does not exist for TRNSYS. The influence of shading on the electrical and thermal energy production can be important and this has to be taken into account in a simulation of a solar thermal system. Unfortunately, there is not a good tool in TRNSYS to calculate the solar radiation on collector fields if the fields are mounted in rows behind each other. In this study the solar radiation is calculated with equations and this requires a lot of work. References 1 Elswijk, M.J.; Strootman, K.J.; Jong, M.J.M. (ECN, Petten (Netherlands)); Lange, E.T.N. de

(Eneco Energie); Smit, W.F. (Eneco Energie), PVT in collectieve zonthermische systemen, ECN-rapport -C--03-112, Petten, The Netherlands, 2003, see http://www.ecn.nl/library/reports/2003/c03112.html

2 TNO; TNO-rapport 2002-DEG-R023, page 47, 12 november 2002 3 Zondag, H.A.; M.J. Elswijk, 3e prototype combi-paneel fase 1, ECN-rapport ECN-C-01-0134,

Petten, the Netherlands, 2001.

4 De Vries, D.W.; Design of a photovoltaic/thermal combi-panel, PhH Thesis Eindhoven University of Technology, Eindhoven, the Netherlands, 1998.

5 Nordmann, T; Clavadetscher, L; 'Understanding temperature effects on PV system

performance', TNC Consulting AG, Switzerland. 6 ISSO publicatie 55; 'Tapwaterinstallaties voor woon- en utiliteitsgebouwen' stichting ISSO,

Rotterdam, the Netherlands, september 2001. 7 Hoiting H.; Boer F. “Zon en Architectuur” Novem, Utrecht, the Netherlands, oktober 2000,

page 131. This study will also be presented as a visual poster presentation on the 19th European Photovoltaic Solar Energy Conference and Exhibition, June 7 -11th 2004, Paris, France.