Gas heat pumps

Introduction

A tried and tested application of mechanical technology to low cost heating and cooling, the heat pump evolved out of cooling applications of the late 1800s, and has developed into technology that can be as reliable as a domestic refrigerator. Heat pumps are a great way to heat buildings and you should consider the use of gas-fired systems as they are cheaper and more efficient than electrically powered ones.

Basic heat pump technology

The technologies used in heat pumps are based on vapour compression systems and absorption systems – both act to move heat from one location to another using energy. Vapour compression systems have developed from the pioneering work of Jacob Perkins in the early 1800s. In the vapour compression cycle, the refrigerant (a fluid which boils and condenses) evaporates into a vapour at a low pressure, by taking in heat at a low temperature (eg, from the outdoor air). The refrigerant vapour is compressed in a mechanical compressor to a higher pressure – this is where the energy is needed – and passed to the condenser where it is condensed back to a liquid releasing heat to provide the source of heating from the heat pump. The liquid then passes through a restriction (such as an expansion valve) to reduce the pressure as it flows into the evaporator. In a vapour compression system, electrical energy is typically consumed by an electric motor driving the compressor (although gas-fired engines can be used).

Edmond Carre patented the first absorption process machine in 1859. Absorption works because some pairs of chemicals have a strong affinity to dissolve in one another. The most common working pairs for absorption systems are:

– water (acting as the refrigerant) and lithium bromide (absorbent) – known as a LiBr-water system, and

– ammonia (acting as the refrigerant) and water (absorbent) – known as an aqua-ammonia system.

The evaporator and condenser are essentially the same as with the vapour compression system, but a chemical absorber and generator replace the compressor, with a pump to provide the pressure change. The pump requires much less power than a compressor and so electrical power consumption is much lower – the heat source provides most of the work to drive the process.

The affinity between the refrigerant and the absorbent is used to draw the refrigerant from the evaporator into the absorber. From there, the weakened solution (ie, one that contains more absorbent) is pumped to a higher pressure, passing through a generator absorber heat exchanger (GAX) to the generator. Heat is then applied, and the refrigerant is driven off to the condenser. The re-strengthened solution can then be recycled to the absorber. Aqua-ammonia systems also incorporate a rectifier (at the top of the generator) and an analyser (between the generator and condenser) to reduce the water vapour being carried with the ammonia refrigerant to the condenser and subsequently into the evaporator.

Absorption devices have been traditionally used for their cooling effect – in particular they have been popular as gas-fired refrigerators (commonly used on boats and caravans) and air-conditioners. The use of absorption systems for heating is a relatively new development.

LiBr-water systems are commonly used in large chiller systems that are hot water or steam driven; aqua-ammonia is widely used in small direct gas-fired chillers and in gas-fired heat pumps. The performance of both types are similar but the evaporator of the LiBr-water system is limited to a minimum of 3 deg C, whereas the aqua-ammonia is capable of operating with evaporating temperatures (ie, the section of the heat pump that is pulling in the heat) below 0 deg C. The LiBr-water system also needs to work at low sub-atmospheric pressures that inevitably leads to air leaking into the system.

Absorption systems have less complex moving parts than a vapour compression system – principally small internal pumps – and so operate with less noise and vibration. The pumps consume much less power than the compressor of a mechanical vapour compression chiller of a refrigeration unit – this is particularly advantageous where there is a limit to the availability of electrical power. The heat to drive absorption refrigerators or chillers can be in the form of hot water, steam, “waste” heat (eg, from an engine) or frequently by direct gas firing. However, currently, absorption heat pumps are only available powered by direct gas-firing and with aqua-ammonia as the working pair.

When a heat pump heats a building, the energy is removed from a low temperature heat source – either the outside air for air-source heat pumps (ASHP) or from a loop embedded in the ground in the case of a ground-source heat pump (GSHP) – and then upgraded in the heat pump to be used to heat air or water inside the building.

Gas absorption heat pumps more efficient

The coefficient of performance for a heat pump when used for heating is the ratio of the output heat (to provide heating or hot water) to the supplied energy (gas or electricity). As the outdoor (source) temperature reduces, the ratio will reduce. This will reduce the output of the heat pump and so at times of peak building heating requirement, when it is coolest outdoors, the heat pump capacity will be at its lowest. This is frequently compensated by incorporating some supplementary electric heating to meet a building’s heating demand at low outdoor temperatures. Not only does this result in higher capital costs, but the carbon impact of the supplementary heating will be higher and in turn more environmentally damaging.

Compared with the vapour compression, absorption heating output will vary much less with the outdoor temperature. The reduction in capacity for an absorption system as the outdoor temperature falls from 5 deg C to -5 deg C is less than 10%, compared with more than 30% for a similar electric heat pump. This means an absorption system can cope with the full design heating load without the need to incorporate supplementary heating (and so save money and energy).

For a gas-fired absorption ASHP, the ratio is likely to be 1.3-1.5 and a typical seasonal ratio will be 1.4. This is about 40% better than a typical gas-fired condensing boiler.

Electric vapour compression heat pumps have a typical theoretical seasonal ratio of 3 and so, in comparison, look an attractive proposition. However, the ratio is not a true measure of the efficiency of the use of fossil fuels and the generation of CO2, as it does not take into account the fuels used, the efficiency of electricity generation at the power station and the losses in electricity transmission around the grid. Utilised electricity from the grid could (depending on location and generation type) produce 2.83 times as much CO2 as natural gas (gross CV gas = 0.185 kg/kWh and rolling 5 year average electricity conversion factor = 0.523 kg/kWh).

This means that although the ratio is higher, an electrically driven vapour compression heat pump may produce 30% more CO2 for the same heating load as a gas-fired absorption heat pump.

Ground-source heat pump systems benefit from a consistent ground temperature so leading to more constant ratio. Unfortunately, the installation cost of ground-source heat pumps – the ground-source heat exchanger and the associated ground works such as drilling or trench digging – makes the systems less attractive. However, as a result of the basic ratio of the gas-fired absorption heat pump being lower than that of the electric vapour compression system, it means that the ground-source heat exchanger required with the gas absorption system is almost half the size of that needed by an electric heat pump for an equivalent load.

Other eco-considerations

The use of a natural ammonia/water mixture as the working fluid instead of the HFC refrigerant working fluids used by electric heat pumps means that the gas absorption heat pump falls outside certain gas regulations. As well as having an ozone depletion potential (ODP) of zero, ammonia has a global warming potential (GWP) of zero with an atmospheric lifecycle of less than a week compared with the current generation of HFCs with GWPs of more than 2000 and atmospheric lifecycles of 30 years or more. Due to the high GWP, an HFC leak from an electric heat pump can undo a large part of the environmental benefits achieved through its high operating efficiencies. Even in the unlikely event of a catastrophic leak from a gas-fired aqua-ammonia heat pump there will be no bad effect on the environment.

Ammonia is a chemically reactive gas that is very soluble in water (hence its use in absorption systems) and is much lighter than air (vapour density 0.59 of that of air). However, cold vapour (eg, from leaks) may be denser than air. Ammonia is characterised by a typical pungent odour and is detectable by most people at levels of about 50 ppm in the atmosphere – a level far below harmful. Ammonia forms a flammable mixture with air at concentrations between 16-25%. Under normal circumstances people will not be able to bear ammonia concentrations at even a fraction of the flammable limit and so there is little practical risk from the ammonia. The amount of free ammonia within an absorption refrigeration system is very limited.

Go for gas

Gas-fired absorption heat pumps can offer an effective low carbon alternative for providing heating and hot water for buildings. The technology is well established with modern gas heat pumps widely used across mainland Europe. They can produce less CO2 than condensing boilers and potentially 30% less than electric vapour compression systems. Where used with ground sources, the area of heat exchanger and associated ground works are smaller than those required for equivalent electric vapour compression systems. The comparative simple mechanical operation of the absorption-based system can provide low noise operation using environmentally benign refrigerants.