Pumping up thermal efficiencies

With gas absorption heat pumps growing in popularity as a renewable alternative to heating and hot water generation, Mike Hefford, Remeha Commercial’s Head of Renewable Technologies, takes a look at the benefits of this lesser known heat pump and its role in decarbonising heating from UK buildings

As the government encourages the decarbonisation of heating for buildings, heat pumps are increasingly specified as a low carbon solution for both new build and refurbishment projects. Far from the “newest pump on the block”, gas absorption heat pumps (GAHPs) are the established, proven technology that is now stepping into the spotlight as a versatile, renewable heating solution offering easy operation, flexible installation, exceptionally high efficiencies and substantial energy and carbon savings.

What is a GAHP?

GAHPs operate similarly to all heat pumps, capturing energy from lower temperature sources, like the surrounding air. They then transfer this useful heat at higher temperatures with the aid of a refrigeration cycle and convert it into useful heat for space heating and hot water. Unlike electric-driven heat pumps, GAHPs use gas-driven heat pump technology and a low-NOx thermodynamic condensing heat generator containing an ammonia water working fluid to draw renewable energy from the air. By combining this energy with the gas input and using the gas condensing heat generator to maintain high operational efficiencies, they are able to increase the thermal output and achieve efficiencies as high as 140% at 65C flow temperature.

High seasonal efficiencies

Historically, heating manufacturers have provided ‘headline’ or best performance efficiency data on their products, which are often difficult to reproduce in real life scenarios. The efficiencies achievable by GAHPs – or indeed any heating technology – are dependent on a number of factors, amongst which the flow/return temperatures. To enable more accurate calculation of the energy performance, we at Remeha have compiled performance tables listing the proven efficiencies of our own Fusion GAHP range at different ambient temperatures and with a variety of flow/return temperatures.

Let’s say GAHPs are to be retrofitted into a building in Manchester. Cross checking the average temperature supplied by the Met office for this region with our own proven performance data brings the following calculations of the expected performance.

North West England/North Wales Average temperature (source: Met Office) Efficiency (GUE) @50C water delivery temperature – heating Efficiency (GUE) @65C water delivery temperature – heating
Autumn 2014 10.9C 155% 131%
Winter 2015 4.1C 147% 118%
Spring 2015 7.5C 152% 124%

The figures reveal that GAHPs reliably deliver exceptionally high, achievable seasonal efficiencies of between 120 and 130%, making them far more efficient even than electric air source heat pumps (ASHP) which can struggle at the lower temperatures.

Easily retrofitted

When it comes to decarbonising heating, our real challenge is to improve the efficiency of our existing inefficient building stock as these buildings will be responsible for 95% of heating consumption in 2020.

An important benefit of GAHPs is that they are suitable for both new build and refurbishment projects. They can be installed as a single unit or in cascade, and easily retrofitted within an existing heating system either as a ‘bolt-on’ to serviceable boilers, or in a hybrid system in conjunction with gas condensing boilers. Further, as they run on gas they require only an extremely low electrical running current of just 1.09 kW for a 35kW GAHP compared with 12.9kW for an equivalent-sized electric heat pump. With no need to increase the electrical incoming supply, there are no extra costs added to installation.

Low operating costs

When evaluating a product’s suitability, the most efficient and economical solution should be evaluated on both its capital costs and running costs. While the purchase price for electric ASHPs and GAHPs is approximately the same, natural gas is typically around a third the price of electricity, resulting in correspondingly lower operating costs for GAHPs for long term fuel savings.

Environmental benefits

When it comes to green credentials, GAHPs score high. Firstly, there’s the carbon factor. By using natural gas as the primary energy source directly at the point of use, GAHPs provide an impressive 98% of usable heat energy compared with the 45% of usable electrical energy of ASHPs which are driven by grid-supplied electricity. This makes them an attractive option for organisations looking to reduce their carbon footprint.

Secondly, they can significantly improve the environmental ratings of a building. GAHPs use an Ammonia/Water solution rather than Hydroflurocarbons (HFC) refrigerants. This brings two BREEAM credits as ammonia has zero global warming potential and ozone depletion potential. The GAHP’s condensing heat generator has a premixed modulating gas burner with low, Class 5 NOx levels (typically 25ppm) for two further BREEAM points. A fifth point is awarded as it is a low carbon technology. GAHPs are also more likely to have a lower operating noise than other renewables in operation.

A case in point

Lake House, an Oxfordshire Care Home, refurbished its heating system with three Fusion 35kW GAHPs in a hybrid system that combined them with 2 Quinta Pro 115kW boilers and a 1,000 litre twin coil buffer through a specially configured BMS. Energy data from the home comparing gas usage from December 2013 to December 14 has revealed a saving of 27% with a corresponding reduction in carbon emissions. This indicates that the care home operators can expect a full return on investment in just five years, independent of any funding or subsidies, with further savings in the future.

Reliable and highly efficient, GAHPs provide a financially and environmentally sustainable solution to low carbon, low NOx heating and hot water provision with rapid payback. This high performance technology, which is fully compliant with the Ecodesign requirements of the ErP Directive, is a practical solution to refurbishing our existing buildings. Given the UK’s high proportion of inefficient building stock, 80% of which still rely on gas for heating, GAHPs represent a practical, affordable means to greater energy efficiency that will assist us in reaching our binding carbon emissions reduction target of 80% by 2050.

This article was originally published in BSEE Magazine.

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