New Low-Temperature Central Heating System Integrated with Industrial Exhausted Heat Using Distributed Electric Compression Heat Pumps for Higher Energy Efficiency

: Industrial exhausted heat can be used as the heat source of central heating for higher energy e ﬃ ciency. To recover more industrial exhausted heat, a new low-temperature central heating system integrated with industrial exhausted heat using distributed electric compression heat pumps is put forward and analyzed from the aspect of thermodynamics and economics. The roles played by the distributed electric compression heat pumps in improving both thermal performance and ﬁnancial beneﬁt of the central heating system integrated with industrial exhausted heat are greater than those by the centralized electric compression heat pumps. The proposed low-temperature central heating system has higher energy e ﬃ ciency, better ﬁnancial beneﬁt, and longer economical distance of transmitting exhausted heat, and thus, its conﬁguration is optimal. For the proposed low-temperature central heating system, the annual coe ﬃ cient of performance, annual product exergy e ﬃ ciency, heating cost, and payback period are about 22.2, 59.4%, 42.83 ¥ / GJ, and 6.2 years, respectively, when the distance of transmitting exhausted heat and the price of exhausted heat are 15 km and 15 ¥ / GJ, respectively. The economical distance of transmitting exhausted heat of the proposed low-temperature central heating system could approach 25.1 km.


Introduction
The rapid development of China's urbanization results in the insufficiency of the heating capacity of existing central heating sources [1]. On the other hand, there are both a great deal of low-temperature exhausted heat available to be the centralized heat source of central heating systems [2], and an overcapacity in China's Northern power grids [3,4]. Recovering low-temperature exhausted heat for central heating by using electric compression heat pumps would help to cover the growing demand of heat load and contribute to increasing the energy efficiency of central heating systems.
Due to the restriction of environmental protection policies, most industry plants are located far away from urban districts. The space mismatch between exhausted heat sources and urban districts would have a greater influence on the thermal performance and the economic benefit of central heating systems based on industrial exhausted heat [5]. For conventional central heating systems based on industrial exhausted heat, industrial exhausted heat is currently recovered by means of a water-to-water heat exchanger [6], or upgraded by using the heat pump [7,8]. Due to the restriction of the shorter economical distance of transporting exhausted heat, the conventional central heating systems integrated with industrial exhausted heat could not recover and transport more exhausted heat to heat users located far away from industry plants [9,10]. Therefore, the longer exhausted heat transportation distance has been a key problem to be resolved for the development of low-temperature central heating systems integrated with industrial exhausted heat.
The longer economical distance of transporting exhausted heat helps to solve the above problem of space mismatch between industry plants and urban districts [11], and also contributes to constructing the heating network of central heating systems based on multi-source energy [5]. By using heat pumps, rising the supply water temperature or reducing the return water temperature of the primary network helps to increase the economical distance of transporting exhausted heat [12,13]. Oluleye et al. [14] present selection principles of heat pumps for higher energy efficiency. Lund et al. [15] point out that introducing the electric compression heat pump into the central heating system contributes to achieving a large socioeconomic potential. Exhausted heat is often upgraded to higher temperature by using centralized electric compression heat pumps [16,17]. Averfalk et al. [18] point out that recovering low-grade exhausted heat for central heating by using centralized electric compression heat pumps helps to improve the performance of cogeneration, and it also improves the flexibility of the heat and electricity supply of cogeneration [19]. The electric centrifugal compression heat pump [20] and the two-stage one [21,22] could upgrade the exhausted heat to 50 • C and 70 • C, respectively. The two-cycle parallel system with centrifugal compressors has a larger coefficient of performance (COP) when the temperature lift is kept at 30 • C [23]. When the temperature lift is 58-72 • C, the COP of the cascade compression heat pump could be improved to 3.1 by using R600 and R290 [24]. The centralized electric compression heat pump can upgrade exhausted heat to higher temperature, but its COP is much smaller. Thus, the application of centralized electric compression heat pumps in recovering low-temperature exhausted heat for central heating is still restricted to a certain extent.
Decreasing the return water temperature of the primary network also contributes to increasing the economical distance of transporting exhausted heat, and helps to recover exhausted heat efficiently [25]. Both the absorption heat exchanger [26] and the ejector heat exchanger [27] can greatly decrease the return water temperature of the primary network in substations, but they need higher-temperature supply water as the driving heat source, which is not lower than 120 • C. Therefore, the two kinds of heat exchanger cannot be applied in the low-temperature central heating systems integrated with industrial exhausted heat. Greatly reducing the return water temperature of the primary network is a key problem to be solved for the low-temperature central heating system integrated with industrial exhausted heat.
To solve the above problem, a new low-temperature central heating system integrated with industrial exhausted heat using distributed electric compression heat pumps (CH-DHP) is proposed.

System Description
For the proposed CH-DHP, electric compression heat pumps are distributed in the substations. In each substation, the distributed electric compression heat pump is coupled with a water-to-water heat exchanger to be a new hybrid heat exchanger unit. The new hybrid heat exchanger unit is illustrated in Figure 1.
As for the new hybrid heat exchanger unit, the supply water of the primary network is first cooled by the return water of the secondary network in the water-to-water heat exchanger, and then further cooled by low-temperature refrigerant in the evaporator of the electric compression heat pump. Return water of the secondary network is first divided into two parts at the inlet. One is heated by circulating water in the primary network in the heat exchanger, and the other is heated by high-temperature refrigerant in the condenser of the electric compression heat pump. The heated water in the secondary network is converged at the outlet and serves as supply water. By this means, the return water temperature of the primary network is greatly decreased. Thus, the temperature difference between the supply and return water of the primary network becomes larger. To clarify features of the CH-DHP, the low-temperature central heating system integrated with industrial exhausted heat using centralized electric compression heat pumps (CH-CHP) and the low-temperature central heating system integrated with industrial exhausted heat using heat exchangers (CH-WHE) are introduced and compared.
In general, a central heating system integrated with industrial exhausted heat consists of a heat source station, the primary network, substations, and the secondary network. For these three central heating schemes, thermal parameters of the secondary network are the same, and thus, the secondary network is not discussed in the following sections.

Operating Principle of CH-DHP
A sketch of the CH-DHP is presented in Figure 2. As for the heat source station, exhausted heat is transferred from industrial exhausted water to the circulating water in the tertiary network by using an anti-corrosive heat exchanger, and it is then used to heat circulating water in the primary network by using a plate heat exchanger. By this means, industrial exhausted heat is recovered efficiently by using a conventional heat exchanger, and it is then transmitted to substations by way of the primary pipelines. In the substations, exhausted heat is transferred from the circulating water in the primary network to that in the secondary network by using new hybrid heat exchanger units, and the return water temperature of the primary network becomes much lower. The lower return water temperature of the primary network contributes to increasing the temperature difference between the supply and return water, and helps to recover more exhausted heat by using conventional heat exchangers in the heat source station.
With the increase in outdoor ambient temperature, heat load demand becomes small during a heating period. For the CH-DHP, the heating capacity of the new hybrid heat exchanger unit can be regulated to cover actual heat load demand by using the variable-frequency drive of the compressor.

Operating Principle of CH-CHP
A sketch of the CH-CHP is illustrated in Figure 3. As for the heat source station, exhausted heat is first transferred from industrial exhausted water to circulating water in the tertiary network via an anti-corrosive heat exchanger, and then it is transferred to that in the primary network by using a conventional plate heat exchanger and a centralized electric compression heat pump. In this way, exhausted heat is recovered by using both the conventional heat exchanger and the centralized electric compression heat pump. By using the centralized electric compression heat pump, exhausted heat can be upgraded to higher temperature. A higher return water temperature of the primary network would result in a higher return temperature of industrial exhausted water, and thus, the exhausted heat recovered decreases. To cover heat load demand, a natural gas-fired boiler is introduced to provide more heat load. The exhausted heat is transported to substations through the primary pipelines. In the substations, exhausted heat is transferred from circulating water in the primary network to that in the secondary network by a water-to-water heat exchanger. In the substations, the return water temperature of the primary network is much higher than that of the secondary network. This is one of the significant differences between the CH-CHP and CH-DHP.
For the CH-CHP, the conventional plate heat exchanger, the centralized electric compression heat pump, and the natural gas-fired boiler are used to regulate the heating capacity of the central heating system to cover the growing demand of heat load as the outdoor ambient temperature rises.
In comparison to the CH-DHP, the CH-CHP features a higher return water temperature of the primary network and lower utilization rate of industrial exhausted heat.

Operating Principle of CH-WHE
A sketch of the CH-WHE is illustrated in Figure 4. The difference in system composition between the CH-CHP and CH-WHE is mainly located at the heating station. In comparison to the CH-CHP, the CH-WHE does not comprise a centralized electric compression heat pump in the heat source station, and its return temperature of industrial exhausted water from the anti-corrosive heat exchanger increases. Thus, the utilization rate of industrial exhausted heat in the CH-WHE is much smaller than that in the CH-CHP.
Comparing with both the CH-CHP and CH-WHE, the CH-DHP is featured by a lower return water temperature of the primary network, higher utilization rate of exhausted heat, and zero consumption of natural gas.

Exergy Calculation
The formula of specific exergy is expressed as [31]: The formula of chemical specific exergy is expressed as follows [32]:

Central Heating System
For the central heating system, the energy conversation equation is written as: During a heating period, the total heat output of the central heating system is calculated as: During a heating period, the total electricity consumption of the central heating system is calculated as:

Evaluation Indicators
During a heating period, the COP of the three schemes varies greatly with outdoor ambient temperature. To clearly clarify the performance of the central heating system, the annual coefficient of performance (ACOP) is presented, and it is defined to be the ratio of annual heat provision to annual electricity consumption, as follows: The annual electricity consumption consists of that of the compressor and water pumps, and the electricity converted from annual natural gas consumption according to the thermal efficiency of current gas-fired power plants of 40%.
To assess exergy performance of these three central heating schemes during a heating period, the annual product exergy efficiency (APEE) is presented, and it is defined to be ratio of annual exergy output to annual exergy input, as follows: The annual exergy output is the exergy difference between supply water and return water of the secondary network. For a central heating system, the annual input exergy comprises electricity, input exergy of exhausted heat, and chemical exergy of natural gas.

Case Study
A case of the low-temperature central heating system integrated with industrial exhausted heat is located in Northern China, and the industrial exhausted water is slag water from the steel plant.

Description of the Case
To analyze the three low-temperature central heating systems integrated with industrial exhausted heat, some information is given as follows: (1) For the given steel plant, the mass flow rate and supply temperature of exhausted water are 265.54 kg/s and 80 • C, and the requirement of the return temperature of waste water is 35 • C.
(2) Heat load is calculated according to both the Design Manual of Central heating [33] and the ratio of heat loss to heating load of 5% [34]. Main thermal parameters of the three central heating systems are listed in Table 1. Table 1 shows that both heat load and temperature difference between supply and return water of the primary network are the same for the three central heating schemes. The potential of recovering exhausted heat in the CH-DHP is greatest, and that in the CH-WHE is smallest among the three central heating schemes. Thus, the role played by the distributed electric compression heat pump in recovering exhausted heat is greater than that by the centralized one. Besides, the COP of the distributed electric compression heat pump is much higher than that of the centralized one.

Thermodynamic Performance
A longer exhausted heat transportation distance would affect the ACOPs and APEEs of the three central heating schemes. The impacts of exhausted heat transportation distance on the ACOPs of three central heating schemes are illustrated in Figure 5.  Figure 5 shows that the ACOPs of the three central heating schemes decrease rapidly with the distance of transporting exhausted heat. It is because the power of water pumps becomes large with the increase in distance of transporting exhausted heat, but heat output is constant. Among the three central heating schemes, the ACOP of the CH-DHP is largest, and that of the CH-WHE is smallest. The ACOP of the CH-DHP is much larger than that of the CH-CHP. It indicates that the role played by the distributed electric compression heat pumps in improving the ACOP of the low-temperature central heating system integrated with industrial exhausted heat is greater than that by the centralized electric compression heat pump. Thus, from the aspect of the ACOP, the CH-DHP is a favorable choice for the low-temperature central heating systems integrated with industrial exhausted heat.
Annual electricity consumption distributions of the three central heating schemes are illustrated in Figure 6.  Figure 6 indicates that when the distance of transmitting exhausted heat is 15 km, the annual electricity consumption of the CH-DHP is smallest, and that of the CH-WHE is largest among the three central heating schemes. The annual electricity consumption of electric compression heat pumps in the CH-DHP is lower than that in the CH-CHP, and the annual electricity consumption of water pumps in the CH-DHP is also lower than that in the CH-CHP. As for the CH-DHP, the annual electricity consumption of water pumps accounts for about 58.5% of total electricity consumption. Besides, there is a certain amount of electricity converted using annual natural gas consumption in the CH-CHP and CH-WHE. In comparison to the CH-CHP, the CH-WHE consumes more natural gas, which could convert more electricity, and it has no electricity consumption of electric compression heat pumps. Thus, recovering exhausted heat by using electric compression heat pumps contributes to reducing annual natural gas consumption, and helps to improve the ACOP of the low-temperature central heating system integrated with industrial exhausted heat.
The impacts of exhausted heat transportation distance on the APEE are shown in Figure 7.  Figure 7 shows that a longer exhausted heat transportation distance results in low APEEs of the three central heating schemes. Among the three central heating schemes, the APEEs of the CH-DHP is largest, and that of the CH-WHE is smallest. With the exhausted heat transportation distance becoming longer, APEE differences among the CH-DHP, CH-CHP, and CH-WHE become smaller. In comparison to the CH-CHP, the CH-DHP has a much higher APEE. Thus, the role played by the distributed electric compression heat pumps in improving the APEE of the low-temperature central heating systems integrated with industrial exhausted heat is greater than that by the centralized electric compression heat pump. From the aspect of the APEE, the CH-DHP is preferred to the low-temperature central heating systems integrated with industrial exhausted heat.
When the distance of transmitting exhausted heat is 15 km, compositions of annual energy consumption of the three central heating schemes are illustrated in Figure 8.  Figure 8 shows that the amount of exhausted heat utilized by the CH-DHP is about 1.06 times of that by the CH-CHP, and about 1.19 times of that by the CH-WHE. During a heating period, the annual electricity consumption of the CH-DHP is lower than that of the CH-CHP, but higher than that of the CH-WHE. During a heating period, the annual natural gas consumption of the CH-WHE is about 3.31 times of that of the CH-CHP, and that of the CH-DHP is zero. During a heating period, the CH-DHP can recover exhausted heat of slag water by about 4.78 × 10 5 GJ. In comparison to the other two central heating schemes, the CH-DHP has a greater potential of energy-saving by recovering more exhausted heat. Thus, the role played by the distributed electric compression heat pumps in improving the thermodynamic performance of the low-temperature central heating systems integrated with industrial exhausted heat is greater than that by the centralized electric compression heat pump.
In summary, the CH-DHP has higher thermodynamic performance, and a greater potential of energy-saving, and thus, its configuration is optimal.

Economic Benefit
Equipment costs are determined by China's current prices, and costs of both installation and maintenance are calculated by the guidance of China's Public Project Investment Estimation [37]. When the exhausted heat transportation distance is 15 km, capital investment distributions of the three central heating systems are shown in Table 2.  Table 2 shows that the capital investment of the CH-DHP is about 0.98 times that of the CH-CHP, and about 1.05 times that of the CH-WHE. The ratio of the primary network's capital investment to total capital investment is about 57.1% for the CH-DHP, about 57.6% for the CH-CHP, and about 61.6% for the CH-WHE. Thus, the capital investment of the primary network is the major composition of total capital investment for the low-temperature central heating system integrated with industrial exhausted heat. A longer exhausted heat transportation distance would result in higher capital investment, and it would have a great influence on financial benefit of the three central heating systems.
In general, heating cost comprises energy cost and nonenergy cost. The nonenergy cost is made up of maintenance cost, labor cost, and the amortization cost of investment capital, and the energy cost consists of waste heat cost, electricity cost, and natural gas cost. According to the life cycle of equipment, the formula of amortization cost is expressed as: where AC, IC, and n are amortization cost, investment cost, and the life cycle of equipment. The value of n is 40 for the heating network, and 15 for the other equipment. For the three central heating schemes, compositions of heating cost are depicted in Figure 9.  Figure 9 indicates that the heating cost of the CH-DHP is lowest, and that of the CH-WHE is highest among the three central heating schemes. With regard to the exhausted heat transportation distance of 15 km, the heating cost of the CH-DHP is about 4.12 ¥/GJ less than that of the CH-CHP, and about 5.08 ¥/GJ less than that of the CH-WHE. The percentage of energy cost in heating cost is about 57.1% for the CH-DHP, about 60.2% for the CH-CHP, and about 63.9% for the CH-WHE.
Payback period is generally used to assess the economic benefit of central heating systems, and its calculating formula is expressed as: where PP, IC, and AP are payback period, investment capital, and annual profit. Impacts of exhausted heat transportation distance on payback period are illustrated in Figure 10.  Figure 10 illustrates that with the increase in exhausted heat transportation distance, the payback periods of the three central heating schemes become long, and payback period differences among the CH-DHP, CH-CHP, and CH-WHE become big. Besides, with regard to the same exhausted heat transportation distance, the payback period of the CH-DHP is always shortest among the three central heating schemes. In accordance with the benchmark payback period of central heating systems, the economical distances of transporting the exhausted heat of the CH-DHP, CH-CHP, and CH-WHE are about 25.1, 20.7, and 19.5 km, respectively. Thus, the economical distance of transporting the exhausted heat of the CH-DHP is much longer than that of the other two central heating schemes.
Comparing with the other two central heating schemes, the CH-DHP has a lower heating cost, shorter payback period, and longer economical distance of transporting exhausted heat. Thus, the role played by the distributed electric compression heat pump in improving the financial benefit of the low-temperature central heating system integrated with industrial exhausted heat is greater than that by the centralized electric compression heat pump. From the aspect of economic benefit, the configuration of the CH-DHP is preferred.

Conclusions
The proposed low-temperature central heating systems integrated with industrial exhausted heat using distributed electric compression heat pumps is studied from the aspect of thermal performance and economic benefit, and conclusions are listed as follows: (1) In comparison to the other two central heating schemes, the proposed low-temperature central heating system integrated with industrial exhausted heat using distributed electric compression heat pumps has higher thermal performance and better economic benefit, and its system configuration is optimal.
(2) Compared with the low-temperature central heating system integrated with industrial exhausted heat using centralized electric compression heat pumps, the proposed one using distributed electric compression heat pumps could improve the ACOP by about 5.12, and APEE by 14.9% when the exhausted heat transportation distance is 25 km.

Conflicts of Interest:
The authors declare no conflict of interest.