ASHRAE OR-16-C046-2016 Coupling PV T Collectors with a Ground-Source Heat Pump System in a Double U-tube Borehole.pdf

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1、Pauline Brischoux is a M.A.Sc. student in the Department of Mechanical Engineering, Polytechnique Montreal, Montreal, Quebec, Canada. Michel Bernier is a professor in the Department of Mechanical Engineering, Polytechnique Montreal, Montreal, Quebec, Canada. Coupling PV/T Collectors with a Ground-So

2、urce Heat Pump System in a Double U-tube Borehole Pauline Brischoux Student Member ASHRAE Michel Bernier, PhD, PE Member ASHRAEABSTRACT This paper examines the possibility of using a double U-tube borehole as a heat exchanger between two independent circuits. One U-tube is linked to a 10 m2 unglazed

3、 photovoltaic-thermal (PV/T) collector and the other to a water-to-water heat pump. The objective of the paper is to quantify the benefits of this proposed system on the seasonal performance factors (SPF) of a ground-source heat pump system used for space heating and domestic water heating of a hous

4、e located in a northern climate. Results show that the proposed system provides 7.7% more electricity than an uncoupled system because the PV/T panels are cooled by the heat transfer fluid from the borehole. However, 81 kWh per year of energy is required to pump this fluid. The heat transferred from

5、 the PV/T panels to the borehole increases the average inlet temperature to the heat pump by about 1.5C which translates into better coefficients of performance (COP) for the heat pump. However, the COP is not the best metric and SPFs, which include pumping energy, represent a better performance ind

6、icator. It is shown that the global value of the SPF increases from 2.82 to 2.88 when the reference system and the proposed system are compared. INTRODUCTION In this article, a double U-tube borehole is used to couple unglazed photovoltaic-thermal (PV/T) collectors and a ground-source heat pump (GSH

7、P) system. The double U-tube acts as a heat exchanger between two independent circuits: one U-tube is linked to the PV/T collectors and the other to the heat pump. This arrangement provides two main advantages. First, the PV/T collectors are cooled which increases the PV cells efficiency and electri

8、city production. Secondly, the ground is thermally recharged which increases the inlet fluid temperature to the heat pump and consequently the coefficient of performance (COP) in heating. The objective of this paper is to quantify, using multi-year simulations, the energy benefits of this proposed c

9、onfiguration for a residential application where a ground-source heat pump system provides space heating and domestic hot water heating (DHW) for a house in a northern climate. LITERATURE REVIEW For ground-source heat pump systems used in cold climates, unbalanced heating/cooling loads result in a r

10、eduction of the ground temperature surrounding boreholes thereby decreasing the heat pump performance. One solution to this problem is to use thermal solar collectors to recharge the ground. Bakker et al. (2005) examined the combination of PV/T collectors with a single U-tube borehole combined to a

11、ground-coupled heat pump. They report that a 25 m PV/T panel produces as much energy as a 26 m thermal solar collector and a 7 m PV panel combined. Furthermore, they observe that heat injection into the ground keeps the ground temperature constant. Trillat-Berdal (2006) showed that by using unglazed

12、 solar collectors to recharge the borehole, the COP of the heat pump is reduced by only 7% over a period of 20 years instead of 9% for a conventional GSHP. Pahud and Lachal (2004) analyzed a system providing space heating and domestic hot water to a single family house in Switzerland. Thermal solar

13、collectors are used to produce DHW and excess solar energy is injected into a borehole. Their results show that the solar panels provide 20% of the heat extracted from the ground during one year, slightly increasing the COP of the heat pump. In addition, coupling the solar panels to the ground preve

14、nts solar collectors from overheating. However, the system requires an additional circulating pump, the electric consumption of which cancels out the energy saved by heat injection. Eslami-Nejad and Bernier (2011) and Eslami-Nejad et al. (2009) coupled thermal solar collectors to a GSHP system using

15、 a four-pipe borehole with two independent U-tube circuits. The results of these studies indicate that the amount of energy extracted from the ground can be reduced by up to 67% using such a system. However, the heat pump energy consumption is only slightly reduced. Kjellsson et al. (2005) concluded

16、 that when thermal solar collectors are used in combination with a ground-source heat pump, it is best to use the thermal solar collectors for DHW heating in the summer and borehole recharging in the winter. Yang et al. (2015) examined experimentally the various possibilities of combining thermal so

17、lar collectors, a storage tank, a heat pump, and boreholes. The highest COP was achieved when the borehole outlet was linked to the solar storage tank and the storage tank outlet to the evaporator inlet and then back to the boreholes. However, pumping energy was not considered in their analysis. Man

18、 et al. (2011) analyzed a PV/T system used in nocturnal cooling mode. They showed that the cooling provided by the PV/T at night was not sufficient to reach the desired temperature. However, the cost of cooling was reduced by about 10% compared to a traditional system. Bertram et al. (2012) conclude

19、d that the use of unglazed PV/T collectors as additional heat source in heat pump systems with borehole heat exchangers increased the PV/T yield by about 4%. The improvement in the value of the seasonal performance factor is 0.36 in the first year and 0.41 for the 20th year of operation. PROPOSED CO

20、NFIGURATION The proposed configuration is presented on Figure 1. It is similar to the configuration used in a companion paper (Hache et al. 2016) except that a PV/T loop and a second U-tube in the borehole have been added to the system. A 10 kW (3 tons) water-to-water ground source heat pump is used

21、 to supply space heating and domestic hot water to a well-insulated 220 m (2368 ft) house. A three-way valve is used to divert the flow from one tank to the other with priority given to the buffer tank for space heating. Figure 1 Schematic diagram of the proposed system with a definition of the SPF

22、boundaries. S P F 5E a u x 2E a u x 1Q au x 2T= 60 CT D H WHea t P u m pE P 1E P 3E P 2E HPS P F 1S P F 2B u f f er t a n kS P F 4S P F 3D HW t a n kQ a ux 1Q H PT B - ta n kM a i n s w a tertem p er a tu r eT inm lo a d.E P4PV /TAAE PVm s o u r c e.A -AP u m p 4P u m p 1P u m p 2P u m p 3When the r

23、eturn temperature from the DHW tank, TDHW, falls below 45C (113F) then the heat pump is started along with pumps P1 and P2. Typically, the top temperature in the DHW tank is around 55C (131F) and a small amount of auxiliary heat is required to reach the desired temperature of 60C (140F). When the ai

24、r temperature in the house drops below 21C (69.8F), pump P3 is activated. If the air temperature continues to drop then an auxiliary heater is energized at 20C (68F) to supplement the heat from the buffer tank. If the temperature in the bottom of the buffer tank, TB-tank, drops below 30C (86F) then

25、the heat pump as well as pumps P1 and P2 are activated. The resulting top temperature in the buffer tank is around 40C (104F) which is typically sufficient to provide about 85% of the annual space heating requirements; the rest is given by the auxiliary heater. Both the source and load flow rates ar

26、e set to 0.28 L/s (4.5 gpm). Characteristics of the main components and operating conditions, including the assumed pressure drops in the various parts of the system, are detailed in Table 1. The GSHP is linked to one of the U-tube of the geothermal borehole. The other U-tube is linked to unglazed P

27、V/T collectors. Both circuits are independent. Four modes of operation are possible: i) Both circuits operate simultaneously and the borehole acts as a heat exchanger between the two circuits; ii) the thermal output of the PV/T is insufficient to recharge the ground and the heat pump is not operatin

28、g so both circuits are inactive; iii) only the heat pump loop operates in which case the borehole takes its normal role of collecting heat from the ground; iv) only the PV/T is in operation to thermally recharge the borehole. The studied building is a typical Canadian single-family house located in

29、Montreal, Quebec. Peak demand for space heating is approximately 8.7 kW (29.7 kBTU/hr) and the annual house space heating requirement is 20800 kWh ( 71 MBTU). The daily domestic hot water consumption is 210 liters (55.5 gallons). The water draw profile is presented in the companion paper (Hache et a

30、l., 2016). On an annual basis, approximately 5000 kWh are required to heat the water from the water mains temperature to 60C (140F). The working fluid circulating on the heat pump side of the borehole is propylene glycol (25%) in accordance to the system presented in the companion paper. On the PV/T

31、 side, methanol with a concentration of 40% is used. Such a high concentration is required due to the exposure of the solar panels to extreme cold weather ( -30C). The use of propylene glycol on the collector side would have led to laminar flow in the U-tube for certain conditions. Table 1. Characte

32、ristics of the Main Components and Operating Conditions Parameter Value Unit , 1034 (2280), 1008 (2222) kg/h (lb/h) / 600 (1320) kg/h (lb/h) P1 (=15%) Pressure drop 72 (236) kPa (ft) P2 (=10%) Pressure drop 16 (53) kPa (ft) P3 (=15% ) Pressure drop 50 (164) kPa (ft) P4 (=10% ) Pressure drop 25 (82)

33、kPa (ft) Volume of DHW and buffer tanks 0.35 (93), 2 (528) m3 (gallons) PV/T Area 10 (108) m (ft) METHODOLOGY The energy performance of the system is assessed with multi-year simulations using TRNSYS (Klein et al. 2010) as the simulation engine. A comparison is made between the proposed system and a

34、 reference system. The reference system is identical to the proposed system except that the PV/T collectors are independent of the rest of the system and the borehole contains only one U-tube linked to the GSHP. Moreover, there is no fluid circulating in the PV/T collectors of the reference system,

35、hence they act as regular PV collectors. Models Standard TRNSYS types are used in the simulation including TYPE741 for calculating pumping energy and TYPE534 for both tanks. The ground source heat pump is modeled using TYPE927 which determines the performance of the heat pump (capacity and power inp

36、ut) by interpolating in a performance map. Figure 2 gives the capacity and COP of the GSHP used here as a function of the entering water temperature on the source side for a fixed value of the load side temperature of 35C (95F). Both the capacity and COP increase with an increase of the entering wat

37、er temperature on the source side. Figure 2 Heat pump performances as a function of the fluid temperature on the source side. Figure 3 Thermal efficiency of the PV/T collectors for different wind speeds. The four-pipe ground heat exchanger model developed by Godefroy (2014) is used here. It is based

38、 on the TRC (Thermal Resistance Capacity) approach, thus fluid and grout thermal capacities are accounted for. Furthermore, it can model two independent circuits such as the ones used here. The borehole characteristics are given in Table 2. Table 2. Borehole Parameters Parameter Value Unit Borehole

39、length 140 (460) m (ft) Buried depth 1 (3.28) m (ft) Borehole diameter (2rb) 0.15 (6) m (in) Pipe outer diameter (2ro) 0.032 (1.25) m (in) Pipe inner diameter (2ri) 0.026 (1.0) m (in) Shank spacing (D) 0.04 (1.57) m (in) Ground thermal conductivity (kground) 2.22 (1.27) W/m.K (Btu/h.ft.F) Grout ther

40、mal conductivity (kgrout) 0.83 (0.48) W/m.K (Btu/h.ft.F) In PV/T collectors, PV cells are mounted on top of an absorber plate with pipes welded to it. Fluid circulates through the pipes thereby collecting thermal energy and cooling the PV cells. In the proposed system, PV/T collectors are simulated

41、using TRNSYS TYPE560 (TESS, 2004) with its default values except for the PV cells efficiency at reference conditions which is set to 15% and the flow rate which is equal to 60 kg/h (132 lb/h). The total surface of the collectors is equal to 10 m (108 ft). The slope of the collectors with respect to

42、the horizontal is 60 to maximize the energy gain during the winter. The electrical and thermal efficiencies are calculated according to the EN 12975-2 standard (2006). Since the collectors are unglazed, the available solar energy per unit surface corresponds to the net irradiance G” (incident solar

43、radiation reduced by longwave radiation). Unglazed solar collectors are relatively sensitive to the wind velocity because the convective heat loss coefficient has a relatively important effect on the thermal efficiency. In the present case, the top loss coefficient is calculated at each time step ac

44、cording to Equation 1, proposed by Duffie and Beckman (2013): = max5,8.6V0.60.4 (1) 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.0800.10.20.30.40.50.6( T m - T a ) / G “ K . m 2 / W Thermal efficiency-V = 1 m /sV = 2 m /sV = 3 m /sV = 4 m /s2 r ok g r o u tk g r o u n d2 D2 r iwhere V is the wind velocity

45、 taken from the weather file and L is the cubic root of the house volume, assumed equal to 8 m. The resulting thermal efficiency of the PV/T collector is shown on Figure 3. These performances are similar to a commercially-available unglazed PV/T collector (Solimpeks, 2015). Pump P4 is activated when

46、 the difference between the PV/T outlet temperature and the average borehole wall temperature is higher than 10C (18F). It is turned off when this same temperature difference becomes lower than 3C (5.4F). Seasonal Performance Factors The efficiency of both the reference and proposed systems is evalu

47、ated using the concept of seasonal performance factors (SPF) which accounts for all the energy exchanges in the system within certain boundaries. These boundaries are identified in Figure 1. The four basic values of the SPF (SPF1 to SPF4) established by Nordman and Zottl (2011) are: 1 = , 2 = + 1, 3

48、 = + 1 + 2 + 1 +1 + 2, 4 = +1 + 2 +1 + 2 + 3 +1 + 2(2a,b,c,d) Values of Q represent annual amounts of heat introduced into the system either from the heat pump, QHP, or from the two auxiliary heating elements, Qaux1 and Qaux2. Values of E represent annual amounts of energy supplied to each component: EHP for the heat pump, EP1, EP2, EP3 for the various circulating pumps, and Eaux1 and Eaux2 for the two electric heating elements. An additional SPF, denoted as SPF5, is proposed here to account for the PV/T loop: 5 = + 1 + 2 + 1 + 2 + 3 + 1 + 2 +(4 )(3) w