Heat pumps for district heat

Industrial heat pumps are credible heat sources for district heating networks. Among the ways that industrial heat pump can be utilized are:

As the primary base load source where a low grade source of heat, e.g. river, fjord, datacenter, power station outfall, sewage treatment works outfall (all typically between 0 ˚C and 25 ˚C) are boosted up the network temperature of typically 60 ˚C to 90 ˚C. Such heat pumps, although consuming electricity, will deliver over 3× and perhaps 5× the heat output as consumed electricity. An example of a district system using a heat pump to source heat from raw sewage is one in Oslo, Norway that has a heat output of 18 MW (thermal).

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 heat pump) or by cooling the closed steam loop and artificially lowering the condensing pressure and thereby increasing the electricity generation efficiency.

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

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

A growing concern exists about the use of hydroflurocarbons as the working fluid (refrigerant) for large heat pumps. Whilst leakage is not usually measurable and is likely to be as low as 1% of total charge, a 30-megawatt heat pump will therefore leak (annually) around 75 kg of R134a or whatever working fluid is deployed. Given the high global warming potential of these HFCs this equates to over 800,000 kilometers (500,000 mi) of car travel per year.

However, recent technical advances allow the use of natural heat pump refrigerants that have very low global warming potential (GWP). CO2 refrigerant (R744, GWP=1) or ammonia (R717, GWP=0) also have the benefit, depending on operating conditions, of resulting in higher heat pump efficiency than conventional refrigerants. An example is a 14 MW(thermal) district heating network in Drammen, Norway which is supplied by seawater-source heat pumps that use R717 refrigerant, and has been operating since 2011. 90 °C water is delivered to the district loop (and returns at 65 °C). Heat is extracted from seawater (from 60-foot (18 m) depth) that is 8 to 9 °C all year, giving an average coefficient of performance (COP) of about 3.15. In the process the seawater is chilled to 4 °C; however, this resource is not utilized. In a district system where the chilled water could be utilized for air conditioning, the effective COP would be considerably higher.

Дата добавления: 2015-08-03