Research on A New District Heating Method Combined with Hot Water Driven Ground Source Absorption Heat Pump

Zhang Qunli[1], Di Hongfa[2], Hu Wangyang[3]
(1 Beijing University of Civil Engineering and Architecture, Beijing, 100044)
(2 Department of Building Science, Tsinghua University, Beijing, 100084)
(3 Chinese Association of Refrigeration, Beijing, 100142)

Abstract:  A new district heating method combined with hot water driven ground source absorption heat pump to extract shallow geothermal was put forward, and the district heating system provides hot water for the driven heat source of the absorption heat pump. By the first energy efficiency and economical analysis method, research results show that compared with the conventional district heating system, the heating capacity of the new district heating system with absorption heat pump can be greatly improved and the heating costs can be largely reduced without expanding thecapacity ofdistrict heating system and increasing other energy consumption. Also, compared with electric-driven ground source heat pump heating system, the new district heating system utilized high temperature water as the driven heating resource to replace the high grade electricity, which can improve significantly the energy utilization efficiency and decrease sharply the power consumption. Compared with electric-driven ground source heat pump heating system on the same heat quantity condition, the new district heating system has the advantage of lower first energy consumption and smaller size of underground heat exchanger and the smaller heat extraction during winter period which was conducive to achieve winter and summerheat balance.  

Keywords: District heating system; shallow Geothermal; Absorption Heat Pump

1 Introduction

The electric-driven ground source heat pump was widely researched and applied to extract shallow geothermal resource for building cooling or heating [1-5]. It relied on electricity for driven resource of heat pump to extract heat from soil for heating in winter and releasing the chiller condenser heat for cooling in summer.

If the electric-driven ground source heat pump fails to maintain seasonal heat balance between winter and summer period, its coefficient of performance will gradually decrease withthe increase of its running time. So the seasonal heat balance of the ground source heat pump is the key point. In addition, electric-driven ground source heat pump in the heating condition consumes much electricity, and the heating cost will depend on the electricity price. So the higher electricity price will be the disadvantage of the electric-driven ground source heat pump system for heating.

A new district heating system combined with ground resource absorption heat pump to extract shallow geothermal resource was presented. The ground resource absorption heat pump is placed in the heat substation to extract heat from the soil or ground water, the district heating system provides hot water with primary heating network for the driven heat resource of the ground resource absorption heat pump. Also, such absorption heat pump can be converted to be absorption chiller for building cooling in summer driven by hot water with district heating system.

Heat exchange facility is applied in heat exchange station in conventional district heating system. Heat exchange equipment only transfers the quantity of heat of the high temperature water of the primary heating network to the secondary heating network, and heat exchange equipment only make use of the thermal of the high temperature water and fails to fully develop the work ability of the high temperature water. Compared with conventional heat exchange equipment in heating substation of district heating system, the absorption heat pump not only makes use of the thermal of the high temperature water but also develops the work ability of the high temperature water, because the absorption heat pump uses the hot water supplied with primary heating network as its driven heating resource to extract the shallow ground resource.

The primary energy efficiency and economic analysis mode of the new district heating method combined with ground source absorption heat pump was set to analyze the difference of the energy efficiency and economy feasibility with conventional system and electric heat pump system. Comparative analysis of district heating system combined with ground source absorption heat pump with electric-drive means for ground source heat pump shows that compared with conventional district heatingsystem, the heating capacity of the new district heating system can be greatly increased and the heating costs can be largely reduced without expanding thecapacity of the heat resource and other power consumption ofdistrict heating system.

Unlike electric-driven ground source heat pump makes use of high-quality electrical to drive, the ground source absorption heat pump system takes advantage of the high temperature water as the driven source, thus the power consumption of the absorption heat pump system will be significantly reduced and then the incremental investment of the new district heating system combined with ground source absorption system can be recovered in four years. Also the ground absorption heat pump system not only combines shallow geothermal with conventional district heating system, but also optimizes the configuration and energy efficiency of districtheat system. Consequently, compared with conventional district heating system or electric-driven ground source heat pump, the new district heating system combined with ground source absorption heat pump system makes significant energy efficient, economic benefits.

2 District heating system combined with ground source absorption heat pump

2.1 System Description

The new district heating mode mainly makes use of ground sourceorwater source absorption heat pump to replace heat exchanger of heat substation in conventional district heating system, and uses high temperature hot water provided by primary heating network as driven heating source for absorption heat pump to  extract heat from low-grade heat sources such as the shallow soil or water, and then both the heat of driving heat source and low-grade heat of ground source are supplied for heating building users by the secondaryheating network.

The new district heating system is mainlyconstitutedwith district heat source (usually a combined heat and power plant, coal or gas heating boiler, etc.), primary heating network, secondaryheating network, distributed ground source absorption heat pump, underground pipes(water taken wells and recharge wells), heating users and their connection accessories.

1.district heat source  2.primary heating supplying water network  3.primary heating returning water pipe 4.ground source absorption heat pump  5.secondary heatingsupplying water network  6.secondary heating returning water network 7.heating users 8.ground supplying water pipe  9.ground returning water pipe 10.Undergroundpipe

Figure 1 Schematic of district heating system combined with ground source absorption heat pump

2.2 Features of the new district heating system

Compared to conventional heating method, the new district heating system combined with ground source absorption heat pump system can increase the heating capacity of the system, reducing heating costs without increasing the capacity of the district heating source. On the other hand, compared with electric-driven ground source heat pump heating method, it is not used high grade electricity as the driven source of heat pump, but made use of the high temperature water supplied by district heating source.Thus it can significantly reduce power consumption and increase the primary energy efficiency of the district heating system. Also, such new district heating approach can more easily achieve the summer and winter seasonal heat balance.  

3 Analysis model of primary energy efficiency  

3.1 Comparison condition setting  

To analyze the energy efficiency and economy feasibility of the new district heating method, the conventional district heating system and the electric-driven heat pump heating system are selected as reference system.

In the condition of the same heating parameter of secondary heating network for building heating users, it is assumed that the buildings for heating users are all energy-saving buildings with low temperature heating parameters and the supplying and returning heating water parameters of secondary heating network is 45/35℃. Schematic diagram of different heating systems and its working parameters are shown as below.

Figure2Conventional district heating system

Figure 3 District heating system combined with ground source absorption heat pump

Figure 4 Electric-driven ground source heat pump system

 

3.2 Primary energy efficiencyevaluation

Primary heating efficiency of different heating system is calculated by equation 1.

            （1）

Which ——Primary heating efficiency

——Heating amount of building users, GJ

——Heating amount of heat source, GJ

——Heating transport efficiency of primary heating network, %

——Heating transport efficiency of secondary  heating network, %

——Heatingefficiency ofcogeneration heat and power system, %

——Heating efficiency of boiler, %

——The power consumption ofprimary heating network pump, GJ

——The power consumption ofsecondary heating network pump, GJ

——The power consumption of underground water pump, GJ

——Electricity generation, GJ

——The average efficiency of power generation of coal power plant

ηe,net——The transport efficiency of power network

The flow charts of primary energy utilizationprocess for different heating systems in the design condition are shown as figure5 to figure 7, and all units in the charts are GJ.

Figure 5Energy flow chart of electric driven ground source heat pump heating system

Figure 6Energy flow chart of conventional district heating system with CHP

 

2.21

Coal
 

Figure 7Energy flow chart of the new district heating system combined with ground source absorption heat pump

3.3 Evaluation results

The primary energy efficiency of different heating systems is shown in figure 8. The figure shows that the primary energy efficiency of the new district heating system combined with ground source absorption heat pump is highest among three heating systems. Compared with the conventional district heating system with combined heat and power heating system, the primary energy efficiency of the new district heating system combined with ground source absorption heat pump can be increased about 20%.

 

 

 

Figure 8 Primary energy efficiency of different heating system in the design condition
 
   

Figure 9ground source absorption heat pump delayheatingload curve
 

Table 1 Primary energy consumption of different heating systems in all heating period

 
    

Units
    

District heating system with ground source absorption heat pump
    

District heating system with CHP
    

Electric- driven ground heat pumpheating mode

Totalheat amount
    

Ten thousand GJ
    

28.2
    

28.2
    

25.4

Supplying heat amount from geothermal
    

Ten thousand GJ
    

5.5
    

0.0
    

19.0

Supplying heat from district heating system
    

Ten thousand GJ
    

22.7
    

28.2
    

0.0

Primary heatingnetworkpump consumption
    

Ten thousand kWh
    

34.6
    

42.8
    

0.0

Secondary heatingnetworkpump consumption
    

Ten thousand kWh
    

101.8
    

101.8
    

91.6

Heat Pump consumption
    

Ten thousand kWh
    

0.0
    

0.0
    

1762.9

Undergroundpump consumption
    

Ten thousand kWh
    

59.2
    

0.0
    

206.3

Totalpower consumption
    

Ten thousand kWh
    

195.5
    

144.6
    

2060.8

Equivalent primary energy consumption
    

Ton standard coal
    

5303.0
    

6192.9
    

7515.2

Primary energy consumptionper unit heating area
    

kgce/m2
    

5.3
    

6.2
    

7.5

 

4 Economic analysis

4.1 Economic model

The initial investment of the new district heating system combined with ground source absorption heat pump is calculated by equation (2).

    （2）

The initial investment of electric-driven ground source heat pump heating system is calculated by equation (3).

               （3）

For cogenerationheating system, the initial investment is calculated as follows.

The initial investment of district heating system with CHP system is calculated by equation (4).

                （4）

The initial investment of the second network, indoor heating pipe network and the end of heating is the same in the above economic model.

The investment base prices of different components are shown in table 3. The cogeneration system is the heat source of district heating system and its investment price can be calculated as 4.5 million RMB / MW (electrical load).

Its power generation efficiency is 25% and its heating efficiency is 50%, and then the ratio of thermal to power is 2.0, thus the initial investment of heating system can also calculated as 2.25 million RMB / MW.

The initial investment price of primary heating network can be calculated as 30 RMB per heating square meter.

The initial investment price of absorption heat pump can be calculated as 0.6 RMB/W (heating load), and the underground pipe can be calculated as 2.5 RMB/W (heat load from underground). The plate heat exchanger and other equipment in conventional substation can be calculated as 0.2 RMB/W. The electric-driven heat pump can be calculated as 1.0 RMB/W and the investment of increasing electricity capacity can be calculated as 1,800 RMB/kW.

The construction and installation costs of the heating system can be calculated as 20% of the equipment initial investment. The construction costs of plant building can be calculated as 30% of the equipment initial investment. The prices of different fuels are shown in Table 2.  

Table2 Fuelprices

 
    

Unit
    

Price

Standard coalprice
    

RMB/tce
    

500.0

Electricity generation price
    

RMB/kWh
    

0.4

Business electricityprice
    

RMB/kWh
    

0.7

Heat price
    

RMB/GJ
    

33.0

 

4.2 Analysis results

As shown in table 3, the installation capacity of the electric-drivenground source heat pump heating system is the smallest because it has no heat loss of primary heating network. Also, as shown in table 4, the investment of the electrically driven heat pump heating system is lowest because of none primary heating network.

The primaryenergy efficiency of the new district heating system combined with ground source absorption heat pump is highest among them, because it can utilize the work capability of the hot water from the district heating system with CHP to extract the shallow geothermal. Also, its annual operating cost is lowest. While, the annual operating cost of the electric-driven heat pump heating system is highest, as shown in table 5.

According to the economics comparative analysis of different heating system, someresultscan be found as follows. Compared with conventional district heating system with CHP system, the investment of the new district heating system combined with ground source absorption heat pump will increase, but its annual operating cost will decrease because its high primary energy efficiency. So its investment recovery period is about 6.9 year.

Compared with the electrical-driven ground heat pumpheating system, the investment of the new district heating system combined with ground source absorption heat pump will increase, but its annual operating cost will decrease, because it can utilize the cheap heat supplied with district heating system with CHP system as its main heat source and also take advantage of the shallow geothermal energy as its auxiliary heat source. So its investment payback period is 4.0 year.

Table 3 Installation capacityof differentheatingsystem

 
    

Basic price
    

Unit
    

District heating system with ground source absorption heat pump
    

District heating system with CHP
    

Electric- driven ground heat pump heating mode

Heat source
    

2.25 million RMB/MW
    

MW
    

32.3
    

40.0
    

0

Increased capacity of power
    

1,800.0 RMB /kW
    

MW
    

1.0
    

0.7
    

10.5

Primary heating network
    

30.0 RMB /m2
    

Ten thousand m2
    

80.6
    

100.0
    

0.0

Increased thermal type units
    

0.6 RMB /W
    

MW
    

32.3
    

–
    

–

Thermal Power Station
    

0.2 RMB /W
    

MW
    

–
    

40.0
    

–

Electric Heat Pump
    

1.0 Yuan/W
    

MW
    

–
    

–
    

9.0

Underground pipe
    

2.0 RMB /W
    

MW
    

7.7
    

0.0
    

27.0

Secondary heating network
    

15.0 RMB /m2
    

Ten thousand m2
    

100.0
    

100.0
    

100.0

Indoor pipe
    

30.0 RMB /m2
    

Ten thousand m2
    

100.0
    

100.0
    

100.0

End of heating
    

40.0 RMB /m2
    

Ten thousand m2
    

100.0
    

100.0
    

100.0

 

 

Figure 10 Initial investment of differentheatingsystem

Figure 11Annual operation costs of differentheatingsystem

Table 4Operation costs of different heating methods (The unit is ten thousand RMB)

Item
    

 
    

Unit
    

District heating system with ground source absorption heat pump
    

District heating system with CHP
    

Electric- driven ground heat pump heating mode

Heat cost
    

Heat cost of district heat system
    

Ten thousand RMB
    

749.1
    

930.6
    

0.0

Pumppowercost
    

Primary heating network
    

Ten thousand RMB
    

13.8
    

17.1
    

0.0

Secondary heating network
    

Ten thousand RMB
    

71.3
    

71.3
    

64.1

Heat pump system
    

Ten thousand RMB
    

0.0
    

0.0
    

1234.0

Undergroundwater pump
    

Ten thousand RMB
    

41.4
    

0.0
    

144.4

Totalheatand electricity cost
    

Ten thousand RMB
    

875.6
    

1019.0
    

1442.6

Operating costsper heating area
    

RMB /m2
    

8.8
    

10.2
    

14.4

Annual operatingcosts (including investment depreciation)
    

RMB /m2
    

20.2
    

21.1
    

24.3

Table 5 Initial investment, operating cost and payback period

 
    

 
    

 
    

Case 1:Referenceobject
    

Case 2 Referenceobject

Item
    

Unit
    

District heating system with ground source absorption heat pump
    

District heating system with CHP
    

Electric- driven ground heat pump heating mode

The initial investment of unit area,
    

RMB/m2
    

231.7
    

221.9
    

209.2

Operating costper unit area
    

RMB/m2
    

8.8
    

10.2
    

14.4

Payback period
    

Year
    

–
    

6.9
    

4.0

 

5 Conclusions

（1）  The new district heating system combined with ground source absorption heat pump to extract shallow geothermal was presented. And it is driven by high temperature water supplied with district heating system.

（2）  Compared with conventional cogeneration district heating system in the same heating capability and heating parameters condition, the primary energy efficiency of the new district heating system combined with ground source absorption heat pump can increase about 22% in the design condition. Also its heating energy consumption can decrease about 15% in the whole heating period.

（3）  Compared with electric-driven ground source heat pump heating system in the same heating capability and heating parameters condition, the primary energy efficiency of the new district heating system combined with ground source absorption heat pump can increase about 42% in the design condition. And the its heating energy consumption can decrease about 30% throughout theheating period.

（4）  Compared with electric-driven ground source heat pump heating system in the same heating amount condition, the new district heating system combined with ground source absorption heat pump extractless heat from ground in winter, so the installation capacity and the initial investment of underground pipes are smaller.

Acknowledgment

Project supported by the National Natural Science Foundation for Young Scholars of China (Grant No 00351911032) and Beijing Municipality Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering.

 

Reference

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