Evaluation of the performance of a centralized ground-water heat pump system in cold climate region

Abstract

The aim of this study is to evaluate the performance of a centralized open-loop ground-water heat pump (GWHP) system for climate conditioning in Beijing with a cold climate in China. Thus, a long-time test was conducted on a running GWHP system for the heating season from December 2011 to March 2012. The analysis of the testing data indicates that the average heat-pump coefficient of performance (COP) and the COP of the system (COPs) are 4.27 and 2.59. The low value and large fluctuation in the range of COP are found to be caused by the heat transfixion in the aquifer and the bypass in the circulation loop. Therefore, some suggestions are proposed to improve the performance for GWHPs in the cold climate region in China.

Keywords

ground-water heat pump (GWHP) / actual coefficient of performance / heat transfixion

Introduction

In the future, the global energy supply must become more sustainable. This can be achieved both by a more efficient use of energy and by relying on renewable sources of energy, particularly wind, hydropower, solar and geothermal energy [1]. A ground-water heat pump (GWHP) system achieves energy conversion by using the solar energy stored in the aquifer as the heat source and sink. This offers many advantages, such as high energy efficiency, and stable operation with minimal impact on environment. Through the use of a small amount of high-grade energy (e.g. electricity), the GWHP unit transfers the low-grade thermal energy to high temperature energy. The temperature of ground water remains 8°C–20°C in summer, and is higher than that of air in winter. Therefore, the GWHP system is the most energy-efficient system in all heat pump systems [2,3].

Many investigations have been proceeding in the design, modeling and testing of GWHP systems. Mustafa Omer [4] has provided a detailed literature-based review of ground-source heat-pump technology, concentrating on loops, ground systems, and presented more briefly at applications, costs and benefits. Stefano Lo Russo et al. [5] carried out numerical simulations and a sensitivity analysis for the subsurface parameters affecting the thermally affected zone. Xue et al. [6] have presented a three-dimensional aquifer thermal energy storage model coupled with a groundwater flow model and a heat transfer model.

Presently, ground-source heat pump systems have been widely used in all 31 provinces in China. More than 500 GWHP projects have been constructed in Beijing till 2007 [7]. Different from the distributed systems in America, the most popular kind of ground-source heat-pump systems is the centralized system in China.

This paper intended to find out the actual operation conditions of GWHP by site test on a commercial building in Beijing, China. The actual energy efficiency level was obtained, and the problems appeared during operation were analyzed and discussed. Then, some suggestions for improving the energy efficiency ratio were proposed.

Project introduction

Project overview

The project is located in Changping District in Beijing. The building, covering an area of 28000 m², with 3 floors and 1 basement whose gross floor area is 40883 m², is a commercial building for car exhibition and spot sale. Beijing, the capital of China, belongs to the cold climate zone in terms of Building Climate Demarcation. Table 1 lists the climatic conditions in Beijing whose annual heating and cooling degree-days are found to be 2794.8 and 70.9 respectively with a base temperature of 18 °C and 22°C.

According to the calculation, the heating and cooling loads of the building are 4258 kW and 5579 kW, respectively. The calculations were conducted under the design outdoor condition and the design indoor temperature of 26°C–27°C in summer and 20°C in winter. A GWHP system was constructed for both heating and cooling. The corrosive nature of the ground water in the area is quite low. The water temperature is 15°C–18°C and remains constant. Therefore, an air conditioning system using underground constant temperature water as the heat source and sink was adopted. The mechanical room was in the basement. The main devices in the system are three large-scale screw-type GWHP units. The terminal units are combination air conditioning units.

Introduction of heat-pump system

Through the exploration and pumping test, groundwater buried depth was found to be 22 m. The yield of a single well is 80 m³/h when the drawdown is 5 m, and the reinjection capacity of a single well is 40 m³/h. In total, 6 wells support the GWHP system, of which, well 1# and 2# are used as the main pumping wells for cooling and heating, respectively. Well 3#, 5# and 6# are used as the recharge wells. Well 4# is a pumping and recharge well (PRW), which is used as the subsidiary pumping well when the building load is comparatively large. Figure 1 shows the schematic diagram of the distribution of the wells.

Three screw-type heat pump units are utilized in this project, whose type and parameters are tabulated in Table 2.

The schematic diagram of the GWHP system is depicted in Fig. 2.

The operation of the underground water source heat pump was as follows. In summer, the underground water of 18°C was extracted by a submersible pump and separated from sand by a hydrocyclone desander. Then it was heated in the condenser of the heat pump unit to 22°C. After that, the water was recharged into the same aquifer. And finally, the cooling water of 7°C produced by heat pump evaporator side was transported to end users. In winter, the underground water was extracted and transported to the heat pump evaporator where the temperature of the water was decreased to 10°C and recharged into the same aquifer. At the same time, the hot water of 45°C produced by the condenser was transported to end users. Four fixed frequency pumps were used as circulating water pumps for the user side and three submersed pumps were used for the well circulation. The parameters of equipment are presented in Table 3.

Measurement system

The test apparatus were set before the heating season in 2011–2012, and the testing data was collected from December 2012 to March 2013. The operating parameters such as the temperatures, flow rates and energy consumptions listed in Table 4 were measured by appropriate instruments.

Results and discussion

The testing data for the whole heating season was obtained by a long-term field test. The operating parameters were arranged and listed in Table 5.

Figure 3 demonstrates the testing data and power consumption data in the cooling season in 2012.

The total power consumption in the cooling season was 252786 kWh and in the heating season was 402069 kWh. Because the heat load is dominating in Beijing, the power consumption in winter is much larger than that in summer. The energy consumption per building area is 6.18 kWh/m² and 9.83 kWh/m², respectively in summer and winter. Besides, it was found that the energy consumption of pumps accounted for 32.11% and 38.59% respectively in the cooling and the heating season.

The energy consumption of the water transportation system should account for 21% to 25% when adopting variable frequency pumps for the well water side [8]. The energy consumption of the transportation system in this project is relatively larger than the reference value. The circulation loop pumps are fixed frequency pumps, while the submersible pumps are manual control variable frequency pumps. The flow control of the circulation loop depends on the numbers of running pumps. This kind of adjustment not only increases the workload, but also leads to high energy consumption in the delivery system.

The heating effect of the heat-pump system could be evaluated by the air temperature in the air-conditioning rooms as

(1)T_{\mathrm{r,s}}={\displaystyle \frac{b_{\mathrm{1}}}{n_{\mathrm{1}}}}{\displaystyle \sum _{i=\mathrm{1}}^{n_{\mathrm{1}}}{T}_{\mathrm{r,}i}}+{\displaystyle \frac{b_{\mathrm{2}}}{n_{\mathrm{2}}}}{\displaystyle \sum _{i=\mathrm{1}}^{n_{\mathrm{2}}}{T}_{\mathrm{r,}i}}+{\displaystyle \frac{b_{\mathrm{3}}}{n_{\mathrm{3}}}}{\displaystyle \sum _{i=\mathrm{1}}^{n_{\mathrm{3}}}{T}_{\mathrm{r,}i}}+{\displaystyle \frac{b_{\mathrm{4}}}{n_{\mathrm{4}}}}{\displaystyle \sum _{i=\mathrm{1}}^{n_{\mathrm{4}}}{T}_{\mathrm{r,}i}},

where b₁, b₂, b₃, and b₄ are the weighting coefficients of the four directions which is east, west, south and north; by statistical analysis, b₁ = b₂ = 16.7%, b₃ = 41.6%, b₄ = 25%; n₁, n₂. n₃, and n₄ are the numbers of temperature sensors in the four directions; and T_r,i is the daily average value of indoor air temperature.

The weighted mean temperature calculated from Eq. (1) was illustrated in Fig. 4. As shown in Fig. 4, the indoor air temperature was increasing along with the environment temperature. The average temperature was approximately 20.6°C and covered the basic needs.

Analysis of heat-pump unit

The main indexes to evaluate the energy-saving performance of ground water heat pump systems are coefficient of performance (COP), seasonal energy efficiency ratio (SEER), and primary energy ratio etc. of which, COP is the most important one.

The useful heat produced by the heat pump unit during the testing period is calculated as

(2)Q=\rho C_{\mathrm{p}}G\left(T_{\mathrm{c},\mathrm{o}}-T_{\mathrm{c},\mathrm{i}}\right),

where ρ is the density of water, C_p is the specific heat of water, G is the water flow in the condenser (m³/h), and T_c,o and T_c,i are the water temperatures of the outlet and inlet of the condenser.

The COP of heat pump unit is calculated as

(3)\mathrm{C}\mathrm{O}\mathrm{P}=Q/N,

where N stands for the input power during the test period.

January is the coldest month of the year, so the analysis was made for one day in January. Figure 5 shows the amount of energy extracted from the groundwater (Q_g), produced by the heat pump unit (Q_h) and inputted to the compressor (P) versus the time in one day. It can be seen from Fig. 5 that the fluctuation range of Q_g and Q_h is 350–450 kW and 450–550 kW while the power input P maintained around 100 kW.

After arranging the hourly testing data of the temperature, the flow rate and power, the hourly COP was calculated according to Eqs. (2) and (3) and exhibited in Fig. 6. It can be seen that the discrete degree of COP was very large. The average value was 4.27, the maximum value was 6.12, and the minimum value was 1.99. Further analysis showed that the sample range was 4.13, the variance was 0.38, the standard deviation was 0.61, and the variation coefficient was 0.14. The minimum COP of the unit was around 9:20, February 27. The reason for such a sudden drop was the increased circulation loop flow. Therefore, the water temperature of the condenser outlet decreased, and the COP significantly dropped. The maximum COP appeared around 10:00 in the morning on March 1st. At that time, the outdoor temperature somewhat rose thus only one unit was in operation. The load factor was as high as 75%. On the other hand, the underground water temperature rose to 18°C, while the supply water temperature dropped to 37°C. These also contributed to the higher level of COP.

Thus, many factors such as the temperature of ground water and supply water could influence the COP of the heat pump unit. Besides, the COP even fluctuated a lot in one day. As shown in Fig. 7, the COP increased to 5.57 gradually, and dropped to a minimum of 3.89 in one day.

During the actual operation, the COP of the GWHP unit was influenced by many factors, including compressor load factor, water temperature of evaporator and condenser, etc. The relationship between the COP and the water temperature of the condenser and the evaporator was indicated in Ref. [9] as

(4)\mathrm{C}\mathrm{O}\mathrm{P}=a-b{T}_{\mathrm{c}\mathrm{d}}-c{T}_{\mathrm{e}\mathrm{v}},

where T_cd and T_ev are the average water temperatures in the condenser and evaporator. respectively, and a, b and c are the regression coefficients.

Thus the temperature of the evaporator and the condenser has a great impact on the COP. When the load factor was 100%, the COP value for different supply temperatures and flow rates were listed in Table 6.

As displayed in Fig. 6, when the temperature of supply water remains the same, the COP decreased with the increase of cooling water flow. Because of the decrease of cooling water flow, the average temperature of cooling water decreased and thus the condensing temperature decreased.

Table 7 shows the variation of COP along with the temperature of evaporator inlet (that is, the well water temperature). It is indicated that the evaporating temperature decreased when well water temperature gradually dropped. Thus the COP of the unit was decreasing gradually.

This GWHP system is a direct system. The groundwater enters the unit after a simple treatment and is directly injected into the same aquifer after the heat transfer. As described above, the PRW 4# started to draw water and replenish the groundwater flow when the building load was large. So there existed a heat transfixion. The injection water was pumped out again without complete heat transfer with the soil. The observation suggested that the variation in the temperature of well water could reach 2°C in summer and 3.5°C in winter in one day.

Analysis of system

The high energy efficiency of GWHP systems depends on not only the high COP of the single heat pump unit, but also the great performance of other components. In contrast to traditional central air conditioning systems, an extra ground water system for heat extracting and exhausting increases not only the initial investment, but also the energy consumption in the delivery system. Therefore, the COP of the whole system was influenced.

The COP of the whole system is defined as the ratio of the refrigerating/heating capacity of the GSHP system to the input power. The input power was generated from the GSHP units and pumps in the system, not including the terminal units.

(5){\mathrm{COP}}_{\mathrm{S}}={\displaystyle \frac{Q_{\mathrm{s}}}{{\displaystyle \sum N_i}+{\displaystyle \sum N_j}}},

where COP_S is the COP of the whole GSHP system, Q_s is the heating capacity (kWh), and N_j is the power consumption of pumps during the test (kWh).

The hourly heating capacity could be calculated by using the hourly average value of the temperatures of supply and return water and circulation loop flow. By adding up the heating capacity of the 24 h, the daily COP_s were obtained, as shown in Fig. 8.

As seen in Fig. 8, the COP_S were generally low and fluctuated remarkably. The maximal value could reach 4.13 while the minimal value was only 0.91, with an average of 2.59. A further analysis of the dispersion of samples showed that the range value of the sample was 3.18, the variance value was 0.58, the standard deviation was 0.76, and the variation coefficient was 0.31. The minimal value appeared on Feb.14. The system was operated at a lower load ratio during the end of the Spring Festival to prevent the circulation loop water from bursting the pipes in severe coldness. Thus, the temperature of the supply and the return water was lower and the loop flow was large.

In general, the COP at partial load is lower than that at full load. Different kinds of units have different COP-load ratio curves. Besides, when several units operate in a combination, the COP_S-load ratio curve is more uncertain due to different operation models and load allocation. The COP_S-load ratio curve for this GWHP system is shown in Fig. 9.

As shown in Fig. 9, the COP of the whole system increased with the load ratio. Thus the lower load factor is one of the main factors causing the low COP_S. In addition, the maximal load factor of this system is only 0.4. It was proved that the design capacity of the GWHP system was quite large.

The generally low and remarkably fluctuated COP_S of this project were found to be caused by the heat transfixion in the aquifer and the bypass in circulation. The heat transfixion in the aquifer has been discussed in Subsection 3.1.

The bypass in the circulation loop was a prominent issue in this project. Beijing is located in the cold climate region, but the heating load is still a little lower than the cooling load. The circulation loop is often designed based on the conditions in summer. Therefore the flow demand in winter is smaller. It was found that the circulation loop flow was always fixed to ensure stable water pressure during operation. The existence of the bypass in the circulation loop caused a certain temperature disparity between the water of the main water supply pipe and the condenser outlet (evaporator outlet in summer). In winter, the existence of the bypass in the circulation loop decreased the temperature of supply water and the temperature difference between the supply and the return water. It also increased the flow rate at lower load ratio, thus the COP_S of the whole system further deteriorated.

Figure 10 shows the temperature of supply water (T_s) and the temperature of condenser outlet water (T_c,o) of the system. As can be observed from Fig. 10, the water temperature difference between the supply water of the system and condenser outlet of the unit was very small. When another heat-pump unit was shut down at noon because of lower load, the temperature of supply water of the system declined significantly. At the same time, temperature difference between the supply and the return water of the system (ΔT) declined to approximately 2.5°C while the temperature difference between the outlet and the inlet of the condenser of the unit (ΔT_c) remained to be 4°C.

Suggestions for GWHP system

According to the conclusions in Section 2, the COP of the heat pump unit and the system were both relatively low and fluctuated. To improve the energy efficiency and save more energy, some suggestions are proposed.

Building load calculation

During the test period, only one unit was in operation most of the time and the load factor was less than 75%. The data obtained also showed that the load factor of the system was always below 50%. This fully illustrates the fact that the design building load is excessively large.

The design building load was calculated simply by unit area indicators. In practical projects, dynamic load simulations should be required, and the air-conditioning devices should be chosen more reasonably.

Well group distribution

Three of the six wells in this project were pumping and recharging wells (PRWs) and the others were recharging wells. Two PRWs were used as the main pumping wells in summer and winter, respectively. And the other PRWs acted as reserve ones, which would be turned on in full load.

However, severe heat transfixion in the aquifer was found in the analysis of the data. Figure 11 shows the variation in the temperature of groundwater water within a day. It can be seen that the well temperature dropped from 18°C to 15°C quickly as a result of heat transfixion. At 12:30, the reserve PRW stopped pumping water with the system load and the requirement of well water flow decreased. Then the temperature of well water increased approximately 3°C–4°C.

When designing, exploration and pumping test should be conducted first. And the numerical simulation of the heat and water transfer in the underground aquifer should be adopted. The PRW well should be avoided when the vertical permeability is relatively high in order to prevent the heat transfixion.

Operation optimization

The system should be operated and adjusted in a “manual control+ automatic” way. Because the load factor of the unit and the frequency of the submersible pumps were controlled manually, the operating condition of the system depends largely on the experience of the operators, which is very ambiguous. This was not conducive to management, and low in labor efficiency.

There exists the serious phenomenon of “small temperature difference but big flow” in the system. For example, the water temperature difference of circulation loop is only about 3°C, while the delivery system consumed over 30% of the total energy consumption. In addition to changing the water pump to automatic frequency conversion, automatic control system is also a powerful way to improve the energy-efficiency ratio and stability. An automatic control system can monitor the running parameters in real-time, alarm fault and ensure operation safety. Moreover, it is able to collect the parameters all the time and save first-hand information for analyzing the existing problems and energy saving calculations.

Conclusions

The GWHP system is one of the fastest growing applications of renewable energy in the world, which represents the fastest development in the utilization of geothermal energy. It provides heating and cooling service in the most energy-efficient way as it uses ground water as the heat source and sink. The ground is a more thermally stable heat exchange medium, particularly unlimited and always available.

In this paper, a long-term field test was conducted in a centralized GWHP system in Beijing, China. The actual performance of a centralized GWHP system in cold climate region was evaluated. The following conclusions can be reached from the analysis in the paper.

1) GWHP systems can provide adequate heat in heating season. The heating effect of GWHP systems is satisfactory. The fluctuation in room temperature is within an acceptable level.

2) The heat-pump COP and the COP of the system are approximately 4.27 and 2.59, respectively. And the distribution of COP value is discrete. This indicates that the operating state of the heat pump system is not steady enough.

3) The heat transfixion and the bypass in the circulation loop are the main problems appeared in operation. The heat transfixion can result in a 3°C–4°C temperature drop in well water supply in winter, and the bypass in the circulation loop can lead to the “small temperature difference and large flow rate” phenomenon.

4) Taking the actual conditions into consideration, dynamic load simulations, well designed well distribution, and optimized control of operating parameters are three effective ways to improve the performance of centralized GWHP systems.

5) The GWHP system is an appropriate heating method. It could be widely used in the cold climate region in China.

References

[1] Bilen K, Ozyurt O,Bakirci K, Karsli S, Erdogan S, Yilmaz M, Comakli O. Energy production, consumption, and environmental pollution for sustainable development: a case study in Turkey. Renewable and Sustainable Energy Reviews, 2008, 12(6), 1529–1561

[2] Sun X G. Engineering Technology and Management of Ground Source Heat Pump. Beijing: China Architecture & Building Press, 2009 (in Chinese)

[3] Zhang Y A, Li B. Application analysis on open-loop surface water-source heat-pump systems. Heating Ventilating & Air Conditioning, 2007, 37(9): 99–104 (in Chinese)

[4] Mustafa Omer A. Ground-source heat pumps systems and applications. Renewable and Sustainable Energy Reviews, 2008, 12(2): 344–371

[5] Stefano Lo Russoa, Glenda Taddiaa , Vittorio Verdab. Development of the thermally affected zone (TAZ) around a groundwater heat pump (GWH. P) system: a sensitivity analysis. Geothermics, 2012, (7): 66–74

[6] Xue Y Q, Xie C H, Zhang Z H, Wu J C. Study on numerical modeling of 3-D aquifer thermal energy storage with transient flow. Geological Review, 1994, 40(1): 74–81(in Chinese)

[7] Lv Y, Mo R, Zhou M, Deng H Y. China GSHP technology application development report (2005–2006). Construction & Design for Project China, 2007, (9): 4–11(in Chinese)

[8] Bai X L, Zhang Y J, Wang H H. Energy efficiency of water transportation system for surface water source heat pump. Journal of Civil, Architectural & Environmental Engineering, 2010, 32(6): 86–91(in Chinese)

[9] Lei F, Hu P F, Huang S Y, Sun Q M. Energy and exergy analysis of a ground water heat pump system. Fluid Machinery, 2012, (2): 57–62(in Chinese)

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg