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Gas absorption benefits drive the technology forward

The stature of gas absorption technology is continuing to grow in the UK as a highly efficient, renewable heating solution for commercial and industrial applications. Stefan Gautsch offers a reminder of how the technology operates and why its popularity is on the rise

The advantages of both ground source and air source heat pumps have been well documented, including their ability to provide sustainable heating and hot water. Gas absorption heat pumps, on the other hand, are a comparatively new concept as far as industry discussion is concerned, but they also offer a proven technology with many benefits.

To summarise, gas absorption heat pumps operate by drawing energy from the air using both a heat pump technology and a highly efficient, gas condensing heat generator to achieve gas utilisation efficiencies of up to 164 per cent. As suggested by the name, this type of heat pump uses gas as the primary energy source directly at the point of use, rather than electricity which is generated largely in coal or gas-fired power stations. By using this technology the gas absorption heat pump has a significantly smaller carbon footprint than comparable heating appliances such as conventional gas-fired boilers.

As well as being able to maximise efficiency, a gas absorption heat pump can also help to significantly reduce running costs. With fuel prices as they are at present, gas is typically only around a third of the price of electricity, which offers direct savings on the fuel source used to generate the heat required. These savings are further enhanced as the system takes advantage of the free energy available in the surrounding air, providing up to 65 per cent additional heat for the building or its hot water demand.

Like their electric counterparts, gas absorption heat pumps are able to extract heat from the air. However, unlike electric heat pumps, there is no requirement for an electrical compressor. Instead, the system uses a generator-absorber heat exchange cycle powered by natural gas or LPG. While all heat pumps require a refrigeration cycle, the gas absorption heat pump uses an ammonia water solution with the ammonia acting as the refrigerant and the water the absorber.

As a renewable technology with low NOx emissions, a gas absorption heat pump qualifies for the upper levels of the BREEAM assessment for sustainable buildings and can help to achieve renewable technology targets required for new build planning permission. A further benefit is that both operating and installation costs are low.

In contrast with many alternatives available for commercial and industrial heating installations, the gas absorption heat pump units are designed for external installation, meaning there is no requirement to take up valuable space in plant rooms, while flue gas systems and bulky fuel storage outlets are not required either.

With regard to retrofit or refurbishment projects, a gas absorption heat pump can add extra value to a building by reducing its running costs and improve the building's energy rating through Energy Performance Certificates (EPCs) and Display Energy Certificates (DECs). As commitments to energy efficiencies such as these become more visible, so will the demand for low carbon technologies of which the gas absorption heat pump is one.

As a low carbon technology, a gas absorption heat pump can offer high efficiencies and carbon saving even when operating at higher temperatures. Maximum flow temperatures of up to 65 deg C can be achieved for heating and 70 deg C for domestic hot water generation. This increases the value of the heat pump itself, in many cases allowing an application's heating to be updated without the need to upgrade associated pipework and associated infrastructure around the building.

Adaptability further enhances the gas absorption heat pump's offering, as it can be easily integrated into an established heating system - taking the place of a condensing boiler, with the benefits of higher carbon savings, reduced environmental impact and lower running costs. The heat pumps can be supplied as pre-fabricated cascades of up to five units, making them suitable for a range of outputs and limiting the requirement for extended on-site installation. The systems can also be integrated with other heating sources such as solar thermal panels and boilers.

Seven key operating principles
The basic operating principles of a gas absorption heat pump can be explained in seven steps (see the illustration above right).

1. Generator. Within the generator the low NOx gas-fired burner heats the ammonia water solution via a heat exchanger, increasing the temperature and pressure. This causes it to separate into a strong ammonia vapour and a weak ammonia solution. The strong ammonia vapour travels to the condenser (2) while the weak ammonia solution is circulated to the absorber (5).

2. Condenser. The now high temperature, high pressure ammonia vapour releases its heat into the heating system in the condenser. In doing so, the vapour changes state and becomes a liquid. This liquid travels to the expansion valve (3) on its way to the evaporator (4).

3. Expansion valve. The ammonia liquid, still at high pressure, passes through the expansion valve where the pressure falls. At this low pressure, ammonia has a reduced boiling point and the liquid changes back to a vapour. This vapour passes on to the evaporator (4).

4. Evaporator. A fan draws ambient air through the fins of the evaporator. The ambient air contains a high amount of free, renewable energy. This energy is captured by the ammonia vapour. The now heated, low pressure vapour passes on to the absorber (5).

5. Absorber. In the absorber, the weak ammonia solution from the generator (1) recombines with the heated vapour from the evaporator (4), having first passed through a second expansion valve (6). As the vapour and weak ammonia solution recombine, the vapour changes state into a liquid, releasing further heat into the heating system. The now recombined ammonia solution is pumped (7) back to the generator (1).

6. Second expansion valve. As described above, this second valve controls the flow of weak ammonia between the generator (1) and absorber (5).

7. Pump. The pump moves the ammonia solution from the absorber (5) back to the generator (1) where the whole process starts again.

// The author is commercial technology consultant for Bosch Commercial and Industrial Heating //
16 April 2013


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