will be equalised by revenue from this wind turbine. Heat price will then only be dependent on investment costs for both the heat pump and wind turbine, and if the wind turbine is not sit- uated on the same site as the heat pump, also electricity tariffs. Marginal heat price can be much lower than the 25 £/MWh for waste heat in the previous examples, but higher investment costs, of course, must be included when compared. Figure 6 shows an example of a comparison of a natural gas reserve- and peak-load boiler, a heat pump alone, and a combined wind turbine and heat pump. The results in this example can vary depending on wind turbine size, compared to heat pump capacity, production profile of the wind turbine and the heat pump, storage capacity, dependency on capacity, etc. How to combine heat sources in future The above examples regarding which heat source is the best compared to electricity and fuel prices show that almost all heat sources can deliver low heat prices if the price conditions are right and heat storage is included in the system for flexibil- ity reasons. Biomass, electrical boilers, and to some extent, natural gas CHP plants may, in the future, be middle-load capacity for heating systems in years with average and low electricity prices. Co-production 3 , infrastructure, ambient, and waste (surplus) 4 heat sources may be base-load. This will switch in years with high electricity prices for the heat sources com- bined with a heat pump, which then will be middle-load. This

heat price security in example figure 4. Low-price waste energy at high temperatures or a biomass boiler is needed, as shown in figure 3. Future heat source design without fossil fuels and biomass The zero carbon targets may not make the above solutions possible because fossil CHP may not be accepted, fossil boilers may only be acceptable for reserve load purposes, and biomass may not be allowed or unavailable. Figure 5 shows the possible scenarios without fossil sources and biomass. In this example, a waste heat base-load source is available, for instance, from a waste incineration plant with a fixed negotiated heat price. When waste heat sources are unavailable and heat can only be produced using a heat pump, the heat price risk is high and almost at the same level as the pure natural gas system shown in figure 2. It is, though, very much dependent on the reserve- and peak-load technology combined with the heat pump. When combining a heat pump and a gas boiler, the heating price can get very high, and the solution is not ideal for heat price security. The only way to equalise the increasing costs of using electric- ity for a heat pump when prices increase is to produce renew- able electricity simultaneously. Suppose the district heating company, dependent on heat production from a heat pump instead of renewable CHP, own a wind turbine at a similar size regarding electricity capacity. In that case, the electricity costs

3 CO-production: Waste incineration, Nuclear heat, Data centre and other industrial sources running 24/7

3 Waste (Surplus): Unreliable surplus heat from industry running in daytime, working days, in seasons, etc.

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

Figure 4. Marginal heat price

natural gas technologies combined with electric heat pump The heat pump delivers a heating price cheaper than natural gas CHP if the electricity price is below 175 £/MWh, and the heating price will not be higher than around 55 £/MWh-heat at this point. In this case, the heat pump should be designed and used for base load when electricity prices are below 175 £/MWh. If electricity prices get above 175 £/MWh, the CHP plant will deliver the main base load capacity. This combination usually can ensure low heat prices.

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