District Heating - Heat Generation

Heat Generation

The core element of a district heating system is as a minimum a heat-only boiler station. Additionally a cogeneration plant (also called combined heat and power, CHP) is often added in parallel with the boilers. Both have in common that they are typically based on combustion of primary energy carriers. The difference between the two systems is that, in a cogeneration plant, heat and electricity are generated simultaneously, whereas in heat-only boiler stations - as the name suggests - only heat is generated.

In the case of a fossil fueled cogeneration plant, the heat output is typically sized to meet half of the peak heat load but over the year will provide 90% of the heat supplied. The boiler capacity will be able to meet the entire heat demand unaided and can cover for breakdowns in the cogeneration plant. It is not economic to size the cogeneration plant alone to be able to meet the full heat load.

The combination of cogeneration and district heating is very energy efficient. A simple thermal power station can be 20-35% efficient, whereas a more advanced facility with the ability to recover waste heat can reach total energy efficiency of nearly 80%.

Other heat sources for district heating systems can be geothermal heat, solar heat, surplus heat from industrial processes, and nuclear power.

Nuclear energy can be used for district heating. The principles for a conventional combination of cogeneration and district heating applies the same for nuclear as it does for a thermal power station. Russia has several cogeneration nuclear plants which together provided 11.4 PJ of district heat in 2005. Russian nuclear district heating is planned to nearly triple within a decade as new plants are built.

Other nuclear powered heating from cogeneration plants are in the Ukraine, the Czech Republic, Slovakia, Hungary, Bulgaria, and Switzerland, producing up to about 100 MW per power station. One use of nuclear heat generation was with the Ågesta Nuclear Power Plant in Sweden closed in 1974.

In Switzerland, the Beznau Nuclear Power Plant provides heat to about 20,000 people.

An important technology in considering the sources of heat for district heating networks are industrial heatpumps. There are several ways that an industrial heatpump can be used, for example:

1-As the primary base load source where a low grade source of heat, e.g. river, fjord, datacentre, power statation outfall, sewage treatment works outfall (all typically between 0C and 25C) are boosted up the network temperature of typically 60C to 90C. Such heatpumps, although consuming electricity, will deliver over 3x and perhaps 5x the heat output as consumed electricity.

2-As a means of recovering heat from the cooling loop of a power plant to increase either the level of flue gas heat recovery (as the district heating plant return pipe is now cooled by the heatpump) or by cooling the closed steam loop and artificially lowering the condensing pressure and thereby increasing the electricity generation efficiency.

3-As a means of cooling flue gas scrubbing working fluid (typically water) from 60C post injection to 20C pre-injection temperatures. The heat is recovered using a heatpump and sold into teh network side of the facility at say 80C.

4-In situations where the network has reached capacity, large individual load users can be decoupled from the feed pipe at say 80C and coupled to the return pipe at say 40C. By adding a heatpump locally to this user, the 40C pipe is cooled to say 20C (the heat being delivered into the heatpump evaporator). The output from the heatpump is then a dedicated loop for the user at say 40C to 70C. Therefore the overall network capacity has changed as the total delta T of the loop has changed from 80-40C to 80C-x (x being a value lower than 40C).

A growing concern exists about the use of Hydroflurocarbons as the working fluids for large heatpumps. Whilst leakage is not ussualy measurable and is likely to be as low as 1% of total charge, a 30MW heatpump will therefore leak (annually) around 75kgs of R134a or whatever working fluid is deployed. Given the high global warming potential of these HFCs this equates to over 800,000km of car travel per year.

Recent advances in technology have allowed the use of natural refrigerants such as CO2 (R744) or ammonia (R717) which also have the added benefits, depending on operating conditions of improved heatpump efficiency.