David Pearson, Director of Innovation at Star Refrigeration discusses the UK’s Renewable Heat Incentive and Star Refrigeration’s successful project of installing the world’s largest ammonia heat pump for district heating in Norway.
With the recent announcement by the British government of the “Renewable Heat Incentive” (RHI), the timing (7 April 2011) of the UK’s “Institute of Refrigeration” evening paper “Ammonia Heat Pumps for District Heating in Norway – a case study” by Kenneth Hoffmann and David Pearson was fortuitous.
The RHI is intended to encourage the deployment of lower carbon systems such as ground and water source heat pumps, as well as other renewable heating solutions such as biofuel combined heat and power.
In the past, it was thought that combining gas or oil fired electricity production and heat recovery was advantageous but the RHI seeks to improve on this technology which has been criticised for achieving lower electrical output than centralised systems whilst locking the early adopters into the combustion of fossil fuels for the next few decades.
It is true that heat pumps rely on electricity as the motive force and that “grid carbonisation” is quite varied across the world but encouraging the user to focus on the local efficiency leaving the national power plants to continue their drive towards a lower carbon network exploits the simple fact that heat pumps draw the majority of their energy source from waste or background sources.
Seawater source NH3 heatpump to increase efficiency and reduce refrigerant charge
Presenting the paper, Kenneth Hoffmann shared the lessons of a practical advantage to harness background heat, in this case ambient seawater. It is appropriate to point out that “seawater source” heat pumps have been successfully utilised in Scandinavia for several decades. However where the system at Drammen differs is that it is designed to be more efficient, encompasses a lower charge and does not utilise HFCs such as R134a. The working fluid is ammonia. Similar systems using HFCs might contain a charge of nearly 10,000 Kgs of R134a. Even a modest leak of 1% per annum would result in very significant global warming potential.
Kenneth Hoffmann described how the partnership between Norsk Kulde of Norway and Star Refrigeration of the UK utilised the high pressure capability of the single screw from Emerson/Vilter to raise heat from 2°C to 90°C. Initial efficiency figures are believed to be considerably in excess of 3.1 meaning that for every 3kWh of heat delivered to the district, less than 1kWh of electricity is harnessed. Even when connected to higher carbon electricity sources such as the UK or Germany this would reduce the carbon effect of heating, but on the hydro electricity networks such as Norway, the system is very close to being zero carbon. Once warmer sources of heat become available, such as waste heat from data centres or low grade geothermal streams at a modest 30°C, the COP will be over 5.0 even for district systems delivering 90°C. For more modest 75°C systems they may be over 6.0.
Lessons to learn from Drammen for the RHI
When similar systems to Drammen are deployed in the UK, the renewable heat incentive would pay a subsidy of 3p / kWh. In the case of a Drammen-scale solution this would deliver a 20 year annual income of up to £3.9M GBP, around €4M per annum. In reality with utilisation and load factors this would be around 1/3 of this.
Aside from the high pressure capability of the Vilter single screw, the control system allows independently variable capacity and volume ratio adjustment. Both of these features are advantageous for district heat pumps as one negates the need for variable speed drives and the other prevents the inefficient internal over compression that would be experienced during differing load patterns.
The utilisation of ammonia takes this already established heating technique one step further. After all, what is the point of reducing fossil fuel related global warming to replace it with synthetic HFC related warming?
Further information is available in the link to Kenneth Hoffmann’s paper.