In symbiosis with the new solar thermal heating plant, the CHP plant becomes more efficient as a back-up for fluctuating wind energy, thanks to investments in expanding the heat storage and a heat pump.

First stage, efficient CHP

Silkeborg District Heating Company is a municipal owned utility. District heating supplies 21,000 buildings and meets approximately 95 per cent of  Silkeborg’s 45,000 citizens heating needs. Two decades ago, the utility invested in Denmark’s largest decentralized gas-fuelled CHP plant (108 MW
electric and 85 MW heat) in order to meet national energy policy objectives. Thanks to co-generation, fuel consumption for heating was reduced compared to indvidual gas boilers.

Second stage, solar and back-up for wind energy

Due to low electricity prices, the CHP plant has been less competitive and production has declined. Moreover, the government and the city council have decided to reduce the consumption of fossil fuels even further. Instead of closing down the plant, it will function as a back up to secure the power supply in periods with no wind and, consequently, higher electricity prices. The first step taken was to install an electric boiler (30 MW) in 2015. The boiler uses the excess wind power and helps balance supply and demand on the electricity market. Symbiosis between solar and CHP The second steps taken were to upgrade the CHP plant and add a large-scale solar heating plant, a larger storage capacity, and a large heat pump. In this symbiosis, the solar heating and the CHP both benefit from the larger storage and the heat pump.

The heat pump (absorption) provides a boost to the heat production from the CHP plant (+30 MW heat) by lowering the temperature from the exhaust (from 65 °C to 23 °C) and increases solar production due to low operating temperatures. The solar plant was the world’s largest when it became operational in 2017. Doubling the heat storage tank capacity from 2 x 16,000 m3 to 4 x 16,000 m3 ensures production fluctuations from the solar heating are accommodated. The storage also makes it possible to increase the  tilization of the CHP plant.

The 25 MW cooling capacity of the absorption heat pump increases production from the solar heating by 15 per cent and increases the total efficiency of the CHP plant via flue gas condensation, from 87 per cent to 102 per cent (based on the lower calorific value).