J. Cent. South Univ. (2012) 19: 1377-1382

DOI: 10.1007/s11771-012-1153-8

Energy efficiency performance of multi-energy district heating and hot water supply system

JIN Nan(金楠), ZHAO Jing(赵靖), ZHU Neng(朱能)

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: A district heating and hot water supply system is presented which synthetically utilizes geothermal energy, solar thermal energy and natural gas thermal energy. The multi-energy utilization system has been set at the new campus of Tianjin Polytechnic University (TPU). A couple of deep geothermal wells which are 2 300 m in depth were dug. Deep geothermal energy cascade utilization is achieved by two stages of plate heat exchangers (PHE) and two stages of water source heat pumps (WSHP). Shallow geothermal energy is used in assistant heating by two ground coupled heat pumps (GCHPs) with 580 vertical ground wells which are 120 m in depth. Solar thermal energy collected by vacuum tube arrays (VTAs) and geothermal energy are complementarily utilized to make domestic hot water. Superfluous solar energy can be stored in shallow soil for the GCHP utilization. The system can use fossil fuel thermal energy by two natural gas boilers (NGB) to assist in heating and making hot water. The heating energy efficiency was measured in the winter of 2010-2011. The coefficients of performance (COP) under different heating conditions are discussed. The performance of hot water production is tested in a local typical winter day and the solar thermal energy utilization factor is presented. The rusults show that the average system COP is 5.75 or 4.96 under different working conditions, and the typical solar energy utilization factor is 0.324.

Key words: geothermal energy; solar thermal energy; district heating; hot water supply

1 Introduction

Building energy consumption takes about 23% in the total energy consumption in China at present and it is in growth trend [1]. Heating loads, cooling loads and hot water loads are the main items of building energy consumption. Renewable energy has been widely used in China for recent years. Geothermal energy and solar thermal energy are most commonly used in buildings. Independent utilization of single renewable energy is popular, but integrated utilization of several kinds of renewable energy is relatively scarce.

A district heating and hot water supply system was designed for the new campus of Tianjin Polytechnic University, China. In this system, geothermal plate heat exchanger (PHE), water source heat pump (WSHP), ground coupled heat pump (GCHP) and natural gas boiler (NGB) are used synthetically to heat the campus buildings; geothermal PHE, vacuum tube array (VTA) and NGB are used complementarily to make domestic hot water.

There are some case studies on geothermal district heating in Turkey in resent years [2-7]. Some researches focused on energy analysis of geothermal heating systems [8-15]. In these researches, deep geothermal energy is the only source of heating. This system uses multi-energy in heating, including deep geothermal energy, shallow geothermal energy and natural gas thermal energy. The system uses solar thermal energy, deep geothermal energy and natural gas thermal energy to make dwells hot water. Though there was previous research on solar energy used in heating and cooling [16], solar hot water used directly as dwells hot water is a direct and highly efficient way to utilize solar thermal energy. The coefficients of performance (COPs) under different heating conditions are discussed according the actual measurement data in the winter of 2010-2011. The performance of solar hot water production in a local typical sunshine winter day is presented. The design idea of this system can be referred in other multi-energy building system designs.

2 Systems and operation strategy

The geothermal, solar thermal and natural gas thermal energy coupling utilization system for district heating and central hot water supply of TPU’s new campus consists of two parts: multi-energy district heating system (MEDH) (Fig. 1) and multi-energy central hot water supply system (MECHWS) (Fig. 2).

Fig. 1 Schematic diagram of MEDHs

Fig. 2 Schematic of MECHWSs

2.1 MEDH

The MEDH utilizes both geothermal energy of deep layer and shallow layer. Natural gas thermal energy is used as an auxiliary and assistant heating source. A couple of geothermal wells that reach the Ordovician in depth of 2 300 m were dug. Each well was set in a well house and equipped with well mouth equipments. Groundwater is drawn up by a variable frequency submersible pump (VFSP). Then, the groundwater is pumped to well water heat exchangers (WWHE) and the hot water heat exchangers (HWHE) of the MECHWSs.

The control valve (CV) in Fig. 1 is only opened when the MECHWS needs geothermal water. The WWHEs are divided into two stages. The groundwater is pumped through the hot side of the first stage of the WWHE (WWHE-I). The cold side of the WWHE-I is connected to the water feeder and collector of the MEDHs. The circular flow pumped by the pump P1 through the cold side of the WWHE-I is a part of the heating flow. The groundwater is divided into two flows after it comes out the hot side of the WWHE-I by an electric control valve (ECV). One part is sent to the hot side of the second stage of the WWHE (WWHE-II) and the flow rate is set according to the outlet water temperature of the cold side of WWHE-II. The outlet water of the WWHE-II is pumped to the evaporators of the WSHP. When the temperature exceeds 24 ℃, the ECV will decrease the groundwater flow rate to the WWHE-II until the temperature is below 24 ℃. When the temperature is below 20 ℃, the ECV will do the opposite operation. This is because exorbitant evaporator water temperature would cause the compressor lubricating oil to be carbonized and the low evaporator water temperature would lead to an inferior COP of the WSHP.

The groundwater flows are combined and sent to the inverted well after cascade utilization, as shown in  Fig. 1. The groundwater can be inverted to 100% due to the local geological structure and there is no need for pressure inverted pumps.

The WSHPs are divided into two stages too, WSHP-I and WSHP-II. The cold side circular flow of the WWHE-II pumped by P2 is just the heat source of the WSHP. The circular flow is sent into the evaporator of WSHP-I and then the evaporator of the WSHP-II gives off heat in sequence. The condensers of the WSHP are paralleled and connected to the water feeder and collector of the MEDHs. The circular flow through the condensers pumped by the pump P3 is another part of the heating flow. The WSHP are also used as freezers in summer with the cold source of cooling towers.

Two GCHPs are paralleled to heat and cool the buildings too. 580 vertical double U-tube ground heat exchangers (VDUGHE) are used to absorb and release heat. The holes of U-tubes are 120 m in depth. The GCHPs are connected to the water feeder and water collector of the MEDHs to supply heating and cooling flows.

Two parallel NGBs are used to assist in heating when the geothermal energy cannot fulfill the peak heat load or the geothermal energy equipments are broken.

2.2 MECHWSs

The MECHWSs utilize solar thermal energy, geothermal energy and natural gas thermal energy in a complementary way. Two parallel VTAs are used to collect solar thermal energy. Each of the VTAs consists of 120 collectors. Each collector has 50 vacuum tubes in parallel connection. Every 5 collectors are set in series and 24 such series are paralleled each other to constitute an array. The total absorber area of two VTAs is       1 500 m².

The solar hot water is used as hot water of dwells directly. The water is circulated by the pump P4 between the VTA and a solar hot water tank (SHWT) which is  20 t in capacity. The water temperatures in the middle height of the tank and the outlet of the VTA are tested by platinum resistance thermometers. The temperature signals are sent to an electric control casing (ECC-1) which controls the water circulation between the VTA and the SHWT.

The water in the SHWT can be pumped by the pump P5 to a central hot water tank (CHWT) which is 200 t in capacity and the water in the CHWT can be pumped to the SHWT by the pump P6. The water temperature in the CHWT is tested by a platinum resistance thermometer. This temperature signal and the SHWT water temperature signal are sent to the second electric control casing (ECC-2). The water circulation between the SHWT and the CHWT is controlled by the ECC-2.

If the collected solar thermal energy cannot meet the energy requirement to make enough hot water of dwells, geothermal energy will be used to heat water through the HWEC. Natural gas thermal energy is also used as an auxiliary and assistant source to heat the water in the CHWT through the HWEC. The water of the CHWT will be circulated by the pump P7 when it needs to be heated through the HWEC. The hot water circulation between the CHWT and the campus building ends is pumped by the pump P8.

The solar thermal energy collected by the VTA cannot be consumed in summer vacation and needs to be stored. The heat load is higher than the cold load of the new campus. There is a need to replenish heat to the soil coupled with the VDUGHE. So, a device was designed and installed for solar-soil heat transformation (SSHT) to resolve the two problems. This device connects the SHWT and 580 VDUGHE. Corrugated heat exchange tubes were set in the SHWT. It was connected to the main inlet and outlet tubes of the VDUGHE. The water circulated by the pump P9 can transfer the redundant solar heat from the SHWT to the shallow soil layer. The heat is stored in the soil and waits for the GCHP exploitation.

2.3 Operation strategy

2.3.1 District heating

The heat load is relatively low in the beginning and the end periods of a heating season. The heat load and electric power consumption of the GCHP are lower than those of the WSHP in the system. So, the WWHE-I and the GCHP are used in parallel in beginning and end heating periods. Only one of the GCHP is used usually, and the other one is used to fulfill the peak heat load.

In the middle period of a heating season, the heat load is always high and the WSHPs are operated instead of the GCHPs. The WWHE-I is always used. The WSHP-I and the WSHP-II are used according to the priority to fulfill the heat load. If the WWHE-I and the WSHP-I could fulfill the campus heat load, the WWHE-II will not be used or be used only to meet the peak heat load.

The VFSP is easy to be damaged in its running surrounding: The deep geothermal well. When it is broken, the WWHE-I and the WSHP cannot be used to supply heat. The GCHP and the NGB will be combined to supply heat at that time.

2.3.2 Hot water production

When the outlet water temperature of the VTA is higher than that of the SHWT at a variable set value in the ECC-1, for example 6 ℃, the ECC-1 can activate the pump P4 to circulate the water. The circulation will be ended when the water temperature difference is under another set value in the ECC-1, for example 3 ℃. The same rule is used in the ECC-2 to control the circulation between the SHWT and the CHWT by activating the pumps P5 and P6. If the CHWT is not full, the ECC-2 can only activate the pump P5 to supply hot water to the CHWT. The SHWT is connected to a tap water pipe. When the water level is lower than half of the SHWT, the ECC-1 will open the solenoid pilot actuated valve (SPAV) on the tap water pipe to supply water to the SHWT.

The above solar thermal energy collection strategy can be used in summer and transition seasons. In such seasons, the water temperature of the SHWT is usually higher than 40 ℃ and it can be used as bath water. But the rule is not applicable in cold winter days. The SHWT water temperature can usually reach 20-30 ℃ under the above rule in sunshine cold winter days. The water temperature of the CHWT is always higher than 40 ℃ by the contribution of geothermal energy and natural gas thermal energy. So, the circulation between the SHWT and the CHWT cannot be activated. If the water consumption in the CHWT is only supplemented by tap water, the collected solar thermal energy will not be used. The water of 20-30 ℃ can be sent to the CHWT, but it must be heated through the HWHE to reach the bath water temperature (at least 44 ℃ in winter). The bath water demand in winter is relatively low and it is more convenient and economic to supply smaller amount with higher temperature solar hot water to the CHWT than larger amount with lower temperature water.

So, the strategy of solar thermal energy utilization in winter is as follows. The auto circulation is ended in the morning and the circulation between the VTA and the SHWT is manually activated when the water temperature of the VTA reaches the top of a day and begins to fall at 14:00 to 15:00. When the inlet water temperature of the SHWT is equal to its outlet water temperature, the circulation is ended manually. The temperature of the SHWT is usually around 50 ℃ at this time. Then, the water in the SHWT is totally pumped to the CHWT. The water in the CHWT is not pumped back to the SHWT in this condition. After the manual operation, the ECC-1 will be set to auto control and tap water will recharge the SHWT. The auto circulation between the SHWT and the VTA will continue until the next morning. In the evening, if the VTA water temperature is lower than the SHWT water temperature by a set value, the ECC-1 will activate the pump P4 to start circulation for freezing prevention.

If the water temperature of the CHWT is not high enough to be supplied as bath hot water, the operators will manually open the CV in Fig. 1 and open the pump P7 in Fig. 2 to heat the water by geothermal energy through the HWHE. The NGB will be used to heat water when the VFSP is broken or the geothermal energy is totally used in heating and has no surplus for hot water of dwells. The NGB is also used as the heat source of HWHE if the VFSP is stopped in summer and transition seasons.

3 Test results and discussion

3.1 Heating performance

COP is defined as the ratio of heating load to electric power consumption. The COP of heating equipments and the whole combination system under different conditions are analyzed according to test data. The geothermal energy utilization factor (GEUF) is defined as the ratio of geothermal heating load to geothermal energy extraction amount. The GEUF of different conditions are analyzed.

3.1.1 WWHE-I and GCHP combined operation

This combination is used from 21:00, Nov. 8th, 2010 to 8:00, Nov. 12th, 2010. The backwater temperature of the heating flow (T1), the outlet water temperature of the WWHE-I cold side (T2), the outlet water temperature of the GCHP condenser (T3), the inlet and outlet water temperatures (T4 and T5) of the WWHE-I hot side were tested by platinum resistance thermometers and the signals were sent to the central computer. T1 is just the inlet water temperature of the GCHP condenser and the cold side of the WWHE-I. The temperatures with the time are shown in Fig. 3. The flow rates of two heating flows and geothermal flow were invariable and tested by a supersonic flow meter. The flow rate of the cold side of the WWHE-I is 444.77 m³/h. It is 279.99 m³/h for the GCHP condenser and 96.538 m³/h for the geothermal water.

The instantaneous heating load of the WWHE-I and the GCHP can be calculated by the above values. The electric consumptions in this heating condition include the VFSP (40 Hz, 56 kW), the circulation pump P1 of the WWHE-I cold side (55 kW) and the GCHP system (498 kW). The heating energy efficiencies are represented by the instantaneous COP of the WWH-I, the GCHP and the combination, as shown in Fig. 4. The average COP of the WWHE-I (COP2) is 13.85. It is 3.95 for the GCHP (COP3) and 5.75 for the combination (COP1). The GEUF is the ratio of the WWHE-I heating load to the geothermal energy extraction amount which can be calculated by T4, T5 and the geothermal flow rate. The result is shown in Fig. 5. The average value of GEUF in this condition is 0.63.

Fig. 3 Heating temperatures as function of time in beginning period of test heating season

Fig. 4 Instantaneous heating efficiencies as function of time in beginning of test heating season

3.1.2 WWHE-I and WSHP-I Combined Operation

This combination was used in the most time of the heating season of 2010-2011. The test results from 8:00, Dec. 14th, 2010 to 21:00, Dec. 17th, 2010 are analyzed here. The same temperatures were tested. T3 is the outlet water temperature of the WSHP condenser. T5 is the inverted geothermal water temperature. The temperatures are shown in Fig. 6. The flow rate of the WWHE-I cold side tested by the supersonic flow meter is 454.72 m³/h in this condition. The flow rate in the WSHP condenser is 404.98 m³/h. The VFSP was set at 45 Hz and the flow rate of geothermal water is 118.33 m³/h.

Fig. 5 Instantaneous GEUG as function of time in beginning of test heating season

Fig. 6 Heating temperatures as function of time in middle period of test heating season

The electric consumptions in this heating condition include the VFSP (45 Hz, 63 kW), the circulation pump P1 of the WWHE-I cold side (55 kW) and the WSHP system (502.9 kW). COP1 of the combination system, COP2 of the WWHE-I and COP3 of the WSHP are shown in Fig. 7. The GEUF (Fig. 8) is the ratio of total heating load to the geothermal energy extraction amount.

The average values of COP1, COP2, COP3 and GEUF are 8.47, 23.43, 4.96 and 1.25, respectively. The high values of COP2 are resulted from the lower electric power consumption of P1 than the GCHP and the WSHP. But the amount of groundwater extraction is limited. So, the usage of the GCHP and the WSHP is necessary. The GEUF is larger because of the geothermal heat promotion by the WSHP.

3.2 Performance of solar thermal energy utilization in MECHWSs

The performance of solar energy utilization is tested in a local typical sunshine winter day, Jan. 19th, 2011, using the strategy described in Section 2.3.2. The peak water temperature of the VTA was at 14:46 and the circulation between the SHWT and one of the two VTAs was manually operated at that time. The water temperature of the SHWT is 24.0 ℃ at the beginning of circulation. When the inlet and outlet water temperatures of the SHWT were equal, the circulation was ended. The stable water temperature of the SHWT was 51.5 ℃ after circulation. Then, all the hot water of the SHWT was pumped to the CHWT. The capacity of hot water was  14 m³ tested by the supersonic flow meter. The heat gain is 1 617 MJ calculated by the above data. It is the heat gain in 6 h of 750 m² VTA and there is 8 h sunshine in a winter day. The local daily solar radiation is 8.865 56 MJ/m² in January according to the data from the Central Meteorological Observatory of China. The solar radiation on 750 m² VTA is 4 986.877 5 MJ in 6 h. The solar thermal energy utilization factor is the ratio of the heat gain to the solar radiation. It is 0.324 in this system and higher than the value of indirect utilization of solar thermal energy which is 0.25 [16].

Fig. 7 Instantaneous heating efficiencies as function of time in middle period of heating season

Fig. 8 Instantaneous GEUG as function of time in middle period of heating season

4 Conclusions

1) A district heating and central hot water supply system was designed for Tianjin Polytechnic University, China. The system uses multi-energy, including deep geothermal energy, shallow geothermal energy, solar thermal energy and natural gas thermal energy.

2) The operation strategy of this system is discussed. Its heating energy efficiency performance was tested in the winter of 2010-2011. The average system COP is 5.75 in the initial period of the heating season with the combination of the WWHE-I and the GCHP. In the middle period of the heating season, the average system COP is 4.96 with the combination of the WWHE-I and the WSHP.

3) The solar thermal energy is directly used as bath hot water and utilization factor is 0.324 tested in a typical cold sunshine winter day.

References

[1] Building Energy Conservation Research Center of Tsinghua University. 2011 Annual report on China building energy efficiency [M]. Beijing: China Construction Industry Press, 2011: 2-3. (in Chinese)

[2] HEPBASLI A, CANAKCI C. Geothermal district heating applications in Turkey: A case study of Izmir-Balcova [J]. Energy Conversion and Management, 2003, 44: 1285-1301.

[3] ERDOGMUS B, TOKSOY M, OZERDEM B, AKSOY N. Economic assessment of geothermal district heating systems: a case study of Balcova-Narlidere, turkey [J]. Energy and Buildings, 2006, 38: 1053-1059.

[4] OKTAY Z, ASLAN A. Geothermal district heating in Turkey: The Gonen case study [J]. Geothermics, 2007, 36: 167-182.

[5] OZGENER L, HEPBASLI A, DINCER I. A key review on performance improvement aspects of geothermal district heating systems and applications [J]. Renewable and Sustainable Energy Reviews, 2007, 11: 1675-1697.

[6] YILDIRIM N, TOKSOY M, GOKCEN G. Piping network design of geothermal district heating systems: Case study of a university campus [J]. Energy, 2010, 35: 3256-3262.

[7] KECEBAS A. Performance and thermo-economic assessments of geothermal district heating system: A case study in Afyon, Turkey [J]. Renewable Energy, 2011, 36: 77-83.

[8] OZGENER L, HEPBASLI A, DINCER I. Energy and exergy analysis of geothermal district heating systems: An application [J]. Building and Environment, 2005, 40: 1309-1322.

[9] OZGENER L, HEPBASLI A, DINCER I. Energy and exergy analysis of Gonen geothermal district heating system, Turkey [J]. Geothermics, 2005, 34: 632-645.

[10] OZGENER L, HEPBASLI A, DINCER I. Performance investigation of two geothermal district heating systems for building applications: Energy analysis [J]. Energy and Buildings, 2006, 38: 286-292.

[11] OKTAY Z, COSKUN C, DINCER I. Energetic and exergetic performance investigation of the Bigadic geothermal district heating system in Turkey [J]. Energy and Buildings, 2008, 40: 702-709.

[12] COSKUN C, OKTAY Z, DINCER I. New energy and exergy parameters for geothermal district heating systems [J]. Applied Thermal Engineering, 2009, 29: 2235-2242.

[13] ARSLAN O, OZGUR M, KOSE R, TUGCU A. Exergoeconomic evaluation on the optimum heating circuit system of Simav geothermal district heating system [J]. Energy and Buildings, 2009, 41: 1325-1333.

[14] HEPBASLI A. A review on energetic, exergetic and exergoeconomic aspects of geothermal district heating systems (GDHSs) [J]. Energy Conversion and Management, 2010, 51: 2041-2061.

[15] KECEBAS A, KAYFECI M, GEDIK E. Performance investigation of the Afyon geothermal district heating system for building applications: Exergy analysis [J]. Applied thermal Engineering, 2011, 31: 1229-1237.

[16] MAMMOLI A, VOROBIEFF P, BARSUN H, BURNETT R, FISHER D. Energetic, economic and environmental performance of a solar-thermal-assisted HVC system [J]. Energy and Buildings, 2010, 42: 15245-1535.

(Edited by YANG Bing)

Foundation item: Project(2010DFA72740-06) supported by International Science & Technology Cooperation Program of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: ZHAO Jing, PhD; Tel: +86-22-27409188; E-mail: zhaojing@tju.edu.cn