Using technologies designed to produce heat 2000 – 4000 hours (middle-load) was not part of the original heat source design. It only emerges if alternative heat sources like industri- al waste heat are found and established or if local authorities choose to build waste incineration plants in networks initially having fossil CHP. When waste heat or incineration deliver heat cheaper than fossil CHP, the CHP unit turns into middle-load capacity if the heat network is not expanded, making heat prices less dependent on fossil prices. Present Heat source design A high amount of renewable power production from wind tur- bines and solar PV plants and the day-ahead electricity mar- ket changes how heat sources should be designed for district heating networks. When the initial heat source design is kept, the fossil CHP plants will produce less heat when electricity prices are low. This makes the reserve- and peak-load boilers produce more heat to cover the decreased heat deliveries from the fossil CHP plant. The heat prices then go up. Fos- sil CHP units can no longer ensure low heat prices, and the district heating companies must look for other solutions and low-price base-load sources than fossil CHP providing low heat prices when the electricity price is low. Heat storage can optimise the income from selling electrici- ty from CHP plants by turning heat production down and electricity production up when electricity prices are high and vice-versa when electricity prices are low. Heat storages also level out heat demand and make production time independ-

ent of heat demand. Heat storage is now beneficial for district heating networks able to sell flexibility to the electricity mar- ket, but it is not enough to prevent heat prices from increasing. The obvious choice is to find a heat sources where the heat- ing price is not dependent on the natural gas or the electricity price. This can be a biomass boiler or an industry’s high-tem- perature waste heat source. Both are limited heat sources not available for everyone and everywhere, but if the district heating network can get access, the source can be important for ensuring low heat prices. Figure 3 shows the variable heat price for gas technologies combined with a cheap waste heat source (25 £/MWh-heat). The design of heat sources could benefit from a heat source delivering low heat prices when electricity prices are low. A heat pump using ambient 1 , infrastructure 2 , and/or low tem- perature waste heat sources found almost everywhere could deliver this. Figure 4 shows how the marginal heat production price develops compared to natural gas sources The present energy crises in Europa due to the Russian/Ukrain- ian war and the following very high natural gas and electricity prices simultaneously show that this solution can also increase the heating prices. If the gas price is 150 £/MWh-gas, the elec- tricity price needs to be below 330 £/MWh-electricity before the heat pump is cheapest. Most importantly, the heating price can go up to 100 £/MWh-heat if both natural gas and electricity prices increase to this level, which means there is no

1 Ambient Heat sources: Air, sea, rivers, lakes, solar, ground water or geothermal sources.

2 Infrastructure: Wastewater treatment, sewage, water pipelines, mines, underground railway, electric transformers, gas compressors, etc.

Marginal heat production costs (Gas price 65 £/MWh)

Figure 3. Marginal heat prices gas technologies combined with a waste heat source

The waste heat source is now the base load tech- nology. The CHP plant is a sort of middle load technology running if the waste heat source cannot deliver enough heat or if electricity prices are very high - above 200 £/MWh. A storage system can opti- mise income from a CHP plant, making the running time independent of heat demand. This ensures a low heating price. Still, both biomass and high-tem- perature waste heat sourc- es can be limited sources only available for few DH networks in the future.

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