Primary Heating Network Research Articles

New district heating system based on natural gas-fired boilers with absorption heat exchangers

Current district heating systems based on natural gas-fired boilers have lower primary energy efficiency due to its higher-temperature exhaust flue gas, and it cannot meet high-density heat load demand by the existing primary heating network. A new district heating system based on natural gas-fired boilers with absorption heat exchangers is presented to increase its primary energy efficiency and meet high-density heat load demand. In this new district heating system, absorption heat exchangers installed in heating substations could greatly decrease return water temperature of the primary heating network. The lower temperature return water could be used to cool exhaust flue gas and increase heat transmission capacity of the existing primary heating network. This new district heating system was analyzed by thermodynamics and economics. Results show that its primary energy efficiency and heat transmission capacity of the primary heating network are increased by about 11% and 47% respectively. When the heat transmission distance of the primary heating network is over 2.6 km, the new district heating system has better thermodynamic performance, economic benefit and environmental benefit, therefore, it would be a better choice for district heating systems based on natural gas-fired boilers with longer heat transmission distance of the primary heating network.

A new waste heat district heating system with combined heat and power (CHP) based on ejector heat exchangers and absorption heat pumps

A new waste heat district heating system with CHP based on ejector heat exchangers and absorption heat pumps (DH-EHE) is presented to decrease heating energy consumption of existing CHP systems by recovering waste heat of exhausted steam from a steam turbine, which could also increase heat transmission capacity of the primary heating network (PHN) by decreasing temperature of the return water of existing PHN. A new ejector heat exchanger based on ejector refrigeration cycle is invented to decrease temperature of the return water of PHN to 30 °C under the designed case. DH-EHE is analyzed in terms of laws of thermodynamics and economics. Compared to conventional district heating systems with CHP (CDH), DH-EHE can decrease consumption of steam extracted from a steam turbine by 41.4% and increase heat transmission capacity of the existing PHN by 66.7% without changing the flow rate of circulating water. The heating cost of DH-EHE is 8.62 ¥/GJ less than that of CDH. Compared to CDH, the recovery period of additional investment of DH-EHE is about two years. DH-EHE shows better economic and environmental benefits, which is promising for both district heating systems for long-distance heat transmission and waste heat district heating systems.

A new ejector heat exchanger based on an ejector heat pump and a water-to-water heat exchanger

As urban construction has been developing rapidly in China, urban heating load has been increasing continually. Heating capacity of the existed primary heating network (PHN) cannot meet district heating requirements of most metropolises in northern China. A new type of ejector heat exchanger (EHE) based on an ejector heat pump and a water-to-water heat exchanger (WWHE) was presented to increase the heating capacity of the existed PHN, and the EHE was also analyzed in terms of laws of thermodynamics. A new parameter, the exergy distribution ratio (EDR), is introduced, which is adopted to analyze regulation characteristics of the EHE. We find that the EHE shows better performance when EDR ranges from 44% to 63%. EHE can decrease the temperature of return water in the PHN to 35°C, therefore, this can increase the heating capacity of existed PHN by about 43%. The return water with lower temperature in the PHN could recover more low-grade waste heat in industrial systems. Because of its smaller volume and lower investment, EHEs could be applied more appropriately in district heating systems for long-distance heating and waste heat district heating systems.

From heat exchanger to heat adaptor: Concept, analysis and application

Based on the analogy between electricity and heat, we find that electric transformers and heat exchangers have many similarities. However, unlike transformers where Vo can be higher than Vi, the outlet temperatures of conventional heat exchangers are limited (i.e. Tc,o<Th,i, Th,o>Tc,i). Therefore, the effectiveness of a heat exchanger cannot exceed unity. Such limitations make it difficult to identify and evaluate thermal equipment or systems for many applications. For example, in city central heating systems, in order to reduce the flow rate of the heat transfer fluid in the primary heat network, m (energy consumption for transporting the fluid is proportional to m3), Th,o is often expected to be lower than Tc,i. In this case a traditional heat exchanger cannot meet this requirement. How to conveniently identify or evaluate the most suitable system for such applications is an important but unsolved problem. In this paper we propose a new concept, the heat adaptor, to address this problem. The idea is to combine heat-work conversion equipment (heat engines and/or heat pumps) with conventional heat exchangers. Using this concept, a typical process of a heat adaptor is established and its thermal performance is analyzed. The results show that the best process and best device arrangement can be obtained for given conditions, and that the thermal performance of these processes can be evaluated.

Study of an innovative ejector heat pump-boosted district heating system

An Ejector heat pump-boosted District Heating (EDH) system is proposed to improve the heating capacity of existing district heating systems with Combined Heat and Power (CHP). In the EDH, two ejector heat pumps are installed: a primary heat pump (HP1) at the heating station and a secondary heat pump (HP2) at the heating substation. With the EDH, the low-grade waste heat from circulating cooling water in the CHP is recycled and the temperature difference between the water supply and the return of the primary heating network is increased. A thermodynamic model was provided. An experimental study was carried out for both HP1 and HP2 to verify the predicting performance. The results show that the COP of HP1 can reach 1.5–1.9, and the return water temperature of the primary heating network could be decreased to 35 °C with HP2. A typical case study for the EDH was analyzed.

A new type of district heating system based on distributed absorption heat pumps

Large district heating (DH) system accounts for 70% of urban building heating in China. In North China, this proportion is even higher (The North China is the north of the Huai River and Qinling Mountains). Many cities in North China can exploit various kinds of low-grade renewable energy. This paper presents a new heating method to realize renewable energy recovery by absorption heat pumps associated with municipal network. In the DH substations, absorption heat pumps are driven by the exergy-difference originated from the larger temperature difference of heat exchange between primary and secondary heat network. There are two configurations—type I and type II substations based on the temperature of renewable energy. A reasonable parameter setting of system is suggested. The equipment operational performance was optimized based on a practical example. The low-grade renewable energy can be recovered effectively in this method. As a result, both heating capacity and energy efficiency of the DH system can be improved. Furthermore, operating costs may be reduced remarkably, due to the reduction in both the coal consumption of heat production unit and the power consumption of delivery pump. Therefore, the system is superior in energy conservation and has a promising application prospect.

Reliability Assessment of Combined Heating System Using Quota Heating Coefficient

The operation reliabilities of heating systems are always of great importance. As to the reliability assessment of a combined heating system with peak load compensation in the secondary heating network, it is suggested to use reliability ensuring coefficient based on quota heating coefficient rather than traditional reliability criteria. It is clear that the reliability ensuring coefficient S is capable of assessing the reliability of combined heating system in the most critical condition. Besides, peak load heat sources in secondary heating network can backup the primary heating network and augment the reliability to a certain extent.

A new type of district heating method with co-generation based on absorption heat exchange (co-ah cycle)

A new type of district heating (DH) method based on Co-ah cycle was designed to improve both the capacity of heating system and the energy efficiency of Cogeneration plant. In the DH system substations, the temperature of return water of the primary heat network is reduced to about 25 °C through the absorption heat-exchange units. In the thermal station of the Cogeneration plant, the return water of the primary heat network is heated orderly by the exhaust steam in the condenser, the absorption heat pumps and the peak load heater. Compared with the traditional DH methods, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. The heating capacity and the energy efficiency of the Cogeneration plant is significantly increased by the exhausted heat recovery from the condensers. A remarkable economic advantage can be achieved in this technology. At present, an application demonstration project had been built up in Chifeng city of Inner Mongolia Autonomous Region in north China.

Analysis of an effective solution to excessive heat supply in a city primary heating network using gas-fired boilers for peak-load compensation

Through investigation of the Dengfeng heating network in the city of Daqing, China, for the 2007–2008 heating season, we found serious problems of excessive heat supply in the primary heating network. Therefore, we propose the application of gas-fired boilers in underperforming heating substations as peak-load heat sources to effectively adapt to the regulation demands of seasonal heat-load fluctuations and reduce the excessive heat supply. First, we calculated the excessive heat supply rates (EHSRs) of five substations using detailed investigative data. We then discussed the feasibility of the proposed scheme providing energy savings from both energetic and exergetic points of view. The results showed that the average EHSR of the five substations between January and March was 20.57% of the gross heat production but consequently reduced to 6.24% with the installation of the gas-fired boilers. Therefore, the combined heating scheme with coal as the basic heat-source and gas-fired boilers as peak-load heat sources is energy-efficient to some extent, although requires the use of natural gas. Meanwhile, the exergy decreased by 10.97%, which indicates that the combined heating scheme effectively reduces the primary energy consumption and pollutant emission of the heating systems.

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.