Abstract Many district heating network companies manage to keep heat prices on the same level in the present European ener- gy crisis with increasing fuel and electricity prices. For con- sumers and the national economy, it can be essential to avoid heat prices following increasing fuel and electricity prices, which can lead to recession, fuel poverty, and affect employ- ment. This article investigates and discusses how it is possible to keep heat prices at the same level when fuel and electricity prices go up, which can inspire the design of future district heat source solutions. The story is about using different tech- nologies complementing each other, often called a Smart Energy system or integrated energy solutions. Original heat source design Most district heating systems operate with a base-load source, which provides the main heat delivery capacity, and a reserve- and peak-load source, which provides capacity able to pro- duce heat when the largest base-load capacity is not running and additionally provide capacity for the very few days in a year, when it is extremely cold, and peak-load capacity is need- ed. The reserve- and peak-load capacity deliver supply security to consumers and additionally ensure low investment costs, as the combination of technologies is cheaper per MW-capacity compared to expensive base low technologies. Originally district heating networks were designed around one large base-load heat source like coal CHP, gas CHP, waste incineration CHP, biomass CHP, and in a few cases, industri- al waste sources. The base-load capacity initially delivers be- tween 50 % and 80 % of capacity (MW) but up to 95 % of the total heat demand (MWh). The share varies from network to

network and depends on local conditions and available heat sources. Figure 1 shows an example of the original design.

The reserve- and peak-load capacity has typically the size of the largest base-load unit and is often an oil or a gas boiler. If only one base-load unit is established, it is usually designed for 70 – 80 % of the total capacity needed, optimizing invest- ments. If the base-load capacity is split between more units, the to- tal base-load capacity can be well above 80 % of capacity de- mand. Then reserve- and peak-load capacity is only needed in the same size as the largest base-load unit. By combining a CHP plant and reserve boiler capacity, the re- serve capacity will also ensure the heating price is not getting too high when the electricity price and earnings from power sales are low. Figure 2 shows how combining a natural gas boil- er, and a natural gas CHP plant can ensure the heating price is not too high. Unfortunately, the system in figure 2 does not always ensure a low heat price because both the electricity price and the heat production price on the boiler depend on the natural gas price. If the natural gas price goes up, the heating price from the boiler also goes up. The heating price can only be main- tained low if the electricity price increases as well, which often is the case in fossil natural gas-based electricity market sys- tems, but not necessarily in an electricity system dominated by renewable electricity sources like water and wind turbines and PV panels.

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

Figure 2. Marginal heat price natural gas technologies

Figure 2 shows that heat production will be based on a natural gas boiler when electricity prices are below 140 £/MWh and on CHP production when above. It is an example including O&M costs and costs for Emission Trade Scheme (ETS) with an estimated 68 £/ton CO 2 . Combining the two technologies ensures that the heat production price will never exceed the natural gas boiler price.

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