Waste-Heat Recovery from Power Plants for District Heating
Applying a centrifugal heat pump and absorption heat pumps to recover waste heat from the cooling water of the power plant steam condenser allowed one city to avoid the cost and emissions of coal-fired district heating.
In 2017, the China Development and Reform Commission issued the Northern China Winter Clean Heating Plan. It mandated that, by the end of 2021, clean combustion of coal in thermal power plants would be used to provide 8 billion square meters of heating area. Estimates say this will account for nearly 40 percent of the total clean-heating area in northern China in 2021.
The plan calls for an increase in the amount of heat supplies from coal-fired thermal power plants. This is achieved by recovering low temperature waste heat from the cooling water of the power plant steam condenser, which is typically cooled by cooling towers. Other clean-heating measures — clean combustion of coal in boilers, natural gas-based heating, electrical heating and heat pumps, renewable energy heating with biofuels, and geothermal and industrial waste-heat recovery — also have been addressed with specific targets.
Shouguang Jintou Heating Co. Ltd. provides district heating to the city of Shouguang, located in the Shandong province. The Chengbei heating station of the company has a total planned heating area of 9 million square meters.
In 2018, the company began to retrofit its district-heating area with energy-saving clean-heating methodologies. During the first phase of the project, approximately 600,000 square meters of heating area were retrofitted with clean heating. This involved recovering low temperature waste heat from the cooling water in the cooling towers of a coal-fired thermal power plant. In subsequent phases of the project, clean heating with various thermal sources such as industrial process cooling water from nearby enterprises, biofuels, geothermal energy, and municipal sewage water are planned.
Shouguang Jintou Heating Co. used the absorption heat pumps driven by steam to deliver hot water to the district heating system. The hybrid thermal and electric heat pumps provide with process flexibility.
For the first phase of the project, Johnson Controls provided two York YHAP-C absorption heat pumps and one York YK centrifugal heat pump. They operate in combination and have a total waste-heat recovery capacity of 17.9 MW in design condition.
Pairing the absorption heat pumps with the centrifugal heat pump helps maximize efficiencies and further reduce the operational cost of the heating system. Traditionally, in power plant waste-heat recovery, steam-driven heat pumps are more popular due to steam availability and restriction on electricity consumption in the power plant. For this project, Shouguang Jintou Heating purchased steam from a power plant at a negotiated (fixed) price to power the absorption heat pumps. Also, because electricity in the city of Shouguang is set at a daily variable rate — dropping to one-third of the price at night compared to daytime rates — the electrical centrifugal heat pump is more cost effective to operate during nighttime hours.
The absorption heat pumps have a two-step evaporator-absorber design, allowing operation with a lower concentration of lithium bromide salt solution. The two-step evaporator-absorber design also uses deionized water as its refrigerant to improve system reliability and durability.
The absorption heat pumps are driven by steam, which is extracted at 116 psi from the steam turbines of the coal-fired thermal power plant. The heat source of absorption heat pumps is from low temperature waste heat in the cooling water from the cooling towers of the coal-fired thermal power plant. It delivers hot water of 167°F (75°C) to the district heating system. The typical heating coefficient of performance (COP) of absorption heat pump is around 1.7.
FIGURE 1. By using both the centrifugal heat pump (pictured) and the absorption pumps, the Shouguang Jintou Heating Co. saved nearly 89,000 tons of water over a four-month period.
The centrifugal heat pump utilizes the R-134a refrigerant and is designed to deliver hot water up to 171°F (77°C). Water with temperature up to 120°F (50°C) can be supplied to the heat pump directly as a heat source, allows the centrifugal heat pump to operate with high efficiency. The typical heating coefficient of performance of the centrifugal heat pump is around 5.0 in thermal power plant cooling water heat recovery for district heating applications. In this project, the centrifugal heat pump is designed to provide hot water of 131°F (55°C), with the heat source water (cooling water from power plant) leaving at 59°F (15°C). The schematic diagram in figure 1 shows the combination system: hybrid absorption and electric heat pump.
The low temperature waste-heat recovery from the cooling tower of the power plant ensures that energy is recovered and not lost to the atmosphere. It also helps save water by reducing the evaporation loss in the cooling tower. The plant benefits from reduced energy consumption and reduced water consumption.
Following the completion of this project, as much as 17.9 MW of heat were recovered by the three heat pumps from the cooling tower. Over the course of the four months that comprise the city of Shouguang’s cold-weather season, the results produced a heat recovery of 175,895 million BTU. To look at it another way, the project provided savings (avoided combustion) of 6,334 tons of standard coal, assuming the standard coal has a heat content value of 27.8 million BTU/ton.
“Recovering low temperature waste heat from cooling towers in thermal power plants by heat pumps is a very efficient clean method of producing heat,” said Pingsheng Yin, the department head at Shouguang Jintou Heating Co. “It significantly reduces the overall fossil fuel and water consumption of the system. The hybrid thermal and electric heat pumps provide us with the necessary flexibility and economic diversity, accounting for changing energy prices. The overall result is a highly efficient, reliable and operationally optimal system.”
The three heat pumps utilized a total cooling water flow rate of 3,083 ton/hour, amounting to 8.88 million tons of water over the four-month period. Because the water was diverted away from the cooling tower, the amount of water saved was 88,798 tons, assuming the avoided evaporation loss is 1 percent in the cooling tower. The heat recovery from the cooling tower, and the resulting energy and water savings, are summarized in the table 1.