District heating is well established in Tallinn, Estonia with approximately 60% of the city being heated from the district heating network. The main source of heat into the network is a Biomass CHP plant using waste wood from local forestry. The moisture content of the woodchip is approximately 50%. In peak periods it has been necessary to start-up gas boilers to provide the total heating capacity. With the goal to reduce their reliance of gas, reduce their carbon footprint and cost of heat they started up a 24 MW heat pump in 2024, which is using the waste heat from the flue gas as heat source. This energy was previously rejected through the chimney, but by recovering the energy and boosting it to 80⁰C so it can be used directly in the district heating network it is possible to reduce the reliance of gas by reducing the consumption with 13.5 million m3, reduce the co2 emissions with 20,000 tons per year and reducing the cost of producing heat. The heat pump work by recovering 18 MW heat from the flue gas by condensing the moisture in the air. With the high moisture content (50%) of the fuel (wood chip) results in a high moisture content of the flue gas. The 4 heat pumps installed operates with a COP above 4, so for every 1 kW of electrical energy used results in 4 kW of heat energy generated. In total 6 MW of electrical energy is used to produce 24 MW of renewable heat. The electrical energy used in from renewable sources giving the heat pump an CO2 emission of zero.
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