Using heat pump to recover waste heat from thermal power plants for district heating

China is now undergoing a fast urbanization, increasing the total district heating (DH) demand. DH systems are gradually expanded and the size of individual system is dramatically growing, leading to a problem of insufficient heat production. Meanwhile, the excess and waste heat of power plant cooling water is not recovered and used efficiently but discarded into the atmosphere. This part of waste heat contains a large amount of energy, for example, Beijing has 6 big combined heat and power (CHP) plants with DH capacity of 4128MW, but they can generate excess heat of 1240MW in the cooling water; the DH production can be increased by 30% if this the excess heat would be recovered.

Although heat pump (HP) technologies are mature in using the low-grade heat sources, the application in power plants is just in the beginning stage. In 2005, the total capacity of coal-fired thermal power plants in China is 3.9×108 kW, among which about 86% is not supplying DH. This means that the cooling towers in those pure thermal power plants are not connected to DH network. The heat loss from cooling towers is equivalent to about 3.4×108 t standard coal. This amount of coal is equivalent to 15% of total energy consumption in China or 36% of the building energy consumption in China in 2005!

The cooling water in the power plants contains a majority of the excess heat and it is better than other low-grade heat sources because of: 1) huge excess heat amount; 2) good water quality; 3) obvious environment protection effect. The means for using excess heat from power plants are: 1) Increase the heat supply without major investment in new heat plants and additional emissions; 2) Solve the heat transportation problem and increase the capacity of DH network; 3) Increase the energy efficiency of power plants and contribute to energy savings

This paper is dedicated to evaluate the different HP technologies for waste heat recovery from thermal power plants and CHP plants, and then find out the most promising scenarios theoretically considering the heat recovery and the connection to DH networks. Different HP technologies will have different connection strategies to DH network, and different operating requirements as well as constraints. The HP technologies studied in this paper include already existing absorption heat pumps, compressor heat pumps and heat pumps under development in laboratory.