DESIGN OF BASE LOAD HEAT SOURCES IN DISTRICT HEATING NETWORKS

By expanding district heating networks, building transmission lines, and intelligent design of heat sources in combination with heat storages fitting to heat demand profiles, it is possible to use waste heat sources 100%. This way, you can avoid losses, get peak load heat demand covered by non-fossil solutions, keep affordable heat prices, and deliver a supply of security to heat consumers, all at the same time.

By John tang Jensen, BEIS

Published in Hot Cool, edition no. 8/2022 | ISSN 0904 9681 |

This article explores how heat sources should be designed for the next generation of district heating networks and how this will benefit consumers and society.

Originally district heating heat source design

When buildings in an urban zone are designated to be supplied from district heating networks, the heat sources commonly are chosen and designed to cover demand and deliver security of supply. It is also designated to deliver low or zero carbon emissions and ensure affordable heat prices by combining heat sources and technologies suitable for different purposes.

Heat sources for district heating were originally mainly based on the waste heat from power production in CHP plants, heat from waste incineration, and in some cases, waste heat from industrial production plants. In most cases, the heat sources existed, and the possible heat delivery was higher than the demand in the district heating network being built for using these waste heat sources. 

Figure 1 shows the combination of waste heat and the needed reserve capacity that district heating networks need to cover the waste CHP supply when this unit is stopped for maintenance or if it falls out.

Figure 1: Duration curve heat demand – ranked from coldest day

The baseload waste heat source can supply all heat in the district heating system, and reserve capacity is only built to ensure the security of supply. The area below the red line and blue shaded area shows the actual delivery of heat covering 98 – 100% of heat demand in the district heating network.

The blue shaded area shows how much more heat the waste heat source could deliver by the installed capacity if delivering is constant at full capacity. The shaded area can easily be up to half the possible heat delivery. If a heat supplier needs to produce power, incinerate waste, or produce industrial goods, the waste heat in the blue-shaded area will be lost, which is not an issue if the price for power, waste, or goods covers costs. The only problem may be the lost energy, which could have been used to reduce carbon emissions and save resources elsewhere in the energy system.

Suppose the power plant, the waste incineration plant, or the industrial plant, due to competition, are getting dependent on the income from heat. In that case, the symbiosis between district heat networks and waste heat suppliers may not work the same way anymore. The heat supplier may need to stop production when heat demand is not present, for example, in the summertime. 

This can be an issue for CHP plants and waste incineration plants, losing the ability to compete on electricity or municipal waste prices if heat cannot be sold. The district heating network company then may not have a reliable and constant baseload supply anymore. This issue can be solved, and the solutions are discussed in the next sections.

Adjusted original heat source design

In most urban areas, district heating networks are not covering all buildings, and some areas may be industrial, using natural gas, which could be replaced by district heating. There may be block-centrals or nearby district heating networks based on boilers or other more expensive heat sources. Heat sales will increase if the district heating network can expand the covered area by connecting more consumers and/or establishing a transmission line to neighbouring networks. 

Then a part of the lost heat in figure 1 can be delivered to consumers without additional investment in production capacity. Figure 2 shows an example of a design where the supply loss is reduced by expanding heat networks.

Figure 2 – Increased delivery and decreased heat loss

The base load capacity, in this case, delivers between 50% and 80% of capacity (MW) but up to 95% of the total heat demand (MWh). The share can vary greatly from plant to plant and depends on local conditions and available heat sources. Compared to the previous example shown in figure 1, the potential heat loss shown in the blue shaded area is reduced by 40% to 70%, depending on the heat demand profile.

This design is very common today. The heat loss is often reduced further if the heat source is a fossil fuel-based CHP plant, not necessarily needing to produce when the electricity price is low and heat demand also is low in the summertime. It often can be beneficial to add a heat storage system making it possible to produce the heat according to electricity prices making electricity production independent of heat demand simultaneously. 

The storage also decreases the need for reserve and peak load heat capacity. It reduces the fuels used for reserve and peak load, which can be important due to low carbon requirements, and to avoid using expensive fuels like oil and gas for peak load purposes.

The blue shaded loss in figure 2 will be more difficult to remove if the waste heat source is constantly producing – from waste incineration plants or from industries.

Design future heat source supply system

The constant running baseload heat capacity needs to be reduced or constructed to around 45% to 55% of the total peak-load heat capacity demand to reduce the blue-shaded loss shown in figure 2 to a very low level. Suppose tap water heating uses 25% of production year-round and heat loss in the network, for example, is 20%. 

In that case, the lowest capacity demand in the summertime will be around 45% of the total demand, which should be the lowest designing point for base load heat sources. Often it can be beneficial to design the base load capacity a little higher, significantly if a storage system can absorb some of the extra waste heat. Figure 3 shows a situation where the base load covers 55% of peak load heat demand.

Figure 3 – Base load heat source design according to capacity demand

When the base load capacity is 55% (MW), the share of heat delivered heat would be around 70% of demand (MWh). Potential heat loss if the base load source needs to run constantly is reduced to a very low level. The original heat source design will not be able to deliver all heat demand in the wintertime if the target is to use peak load source as little as possible.

The heat source design then needs a low carbon “middle load” source to deliver heat in the wintertime. This can be a heat pump using air, other ambient sources, or low-grade heat waste heat from infrastructure sources – municipal wastewater treatment, water systems, Transformers, underground trains, gas compressors, mines, etc. – or allowed biofuels. 

The choice of middle load technology should complement the base load technology or at least not be dependent on the same fuel. If baseload technology is CHP-dependent on high electricity prices, it would be a good choice to choose a middle-load technology dependent on low electricity prices, like heat pumps using ambient or low-grade waste infrastructure heat sources.

The capacity of these middle-load technologies can be higher than the expected 40%, as shown in figure 3 if higher, the middle load capacity can deliver peak low capacity and additionally be able to deliver reserve load capacity for the base load unit. 

This way, it can reduce fossil peak load capacity to zero. It additionally can be recommended to design these middle load source technologies in combination, maybe both having a heat source using a heat pump, a waste heat source, and/or a biomass boiler. 

If the power system needs power capacity, even CHP solutions could be considered. The combination of technologies can ensure low heat prices because the technology getting more expensive by increasing prices can be turned down and other technologies turned up

Design of new district heating networks

Two main approaches can be considered when designing new networks and heat sources for new networks.

If a large existing waste heat source is already available, it would be convenient to start delivery from this source the same way as the original heat source design. Focus should then be on expanding the heat network until the heat source design needs to be adjusted and supplemented with middle load sources. 

In the network expanding phase, the chosen reserve load technologies should be suitable for middle load and, in the beginning, maybe only used for peak and reserve load purposes. In the end, the heat network demand may reach a level having a heat source design like the future design shown in figure 3.

If no large existing waste heat source is available from the beginning, another approach may be better and recommendable. Often it takes time to get consumers connected in new networks, and it can then be recommended to start up with the middle load technologies, also providing base load capacity when the heat network is being built. 

This gives time to find a better high-grade waste base load technology that can take over with full capacity from finished construction a little later. 

This will make base load suppliers get the expected sales and revenue from the beginning. If no waste heat sources are available for base load in the new network area, this way of designing heat sources gives time to attract, for example, a new waste incineration plant to an area or to attract data centres, hydrogen production plants, Power-to-X, all wanting to run constantly and deliver full load heat capacity. 

Especially for waste incineration plants and large, continually running waste heat suppliers, high heat delivery is essential and can trigger incentives for establishing solutions for delivering waste heat. The feasibility simply gets better when supply can be expected full-time, and no heat is wasted like the blue shaded areas shown in figures 1 and 2.

The middle load technologies, which were delivering all heat from the start, will now be able to deliver heat in the wintertime, deliver flexibility to the electricity system if based on electricity and/or CHP, and ensure low heat prices. 

This is because the production can be changed according to electricity and fuel prices. If a heating system is constructed the right way, including heat storage, it will work the same as a battery, which can be very valuable for society and the electricity system saving capacity and balancing costs.

For further information please contact: John Tang Jensen, JohnTang.Jensen@beis.gov.uk