Absorption Technology

How Low Carbon Cooling and Heating Works
The technological basis of both the Yazaki and Robur product ranges supplied by Shorts is the absorption heat pump system.

Heat pumps do not generate heat in the same way that a gas-boiler burns gas to generate heat – they are devices that use a closed loop fluid circuit to move heat from one place to another. Heat pumps can provide heating or cooling or even heating and cooling from the same unit. The most commonly used heat pump, known to everyone, is a refrigerator. This is simply a cooling only heat pump that removes heat from the inside of the ‘fridge and dumps it outside. The thermodynamic process is such that the level of the “waste” heat is at a higher temperature – that’s why the back of your fridge is always warm ! This same principle is used in all building air conditioners or chillers.

Taking this concept on a stage, it is easy to see how a heat pump can heat a building. A heat pump in heating mode can take heat from the outside air (air source) or even from the ground (ground source), upgrade it and reject it to the inside of the building. This process works even if the outside air is cold. Despite the temperature of ambient air it will always contains some level of heat. The same unit, put into reverse mode (chiller) operation can also provide cooling for the inside of the building.

As with refrigerators, the traditional way of powering heat pumps is via electricity. The electricity drives a compressor which circulates the working fluid around the closed loop system of the heat pump. This is known as a vapour compression heat pump or chiller. An alternative system uses relatively high temperature heat to drive the working fluid around the circuit using a secondary cycle and an absorbent fluid. This system is known as an absorption heat pump or chiller. A simple gas-fired absorption chiller or heat pump contains most of the components that you would find in a conventional vapour compression device. The “driving” heat for the absorption machine can be in the form of hot water, steam or even direct firing by gas.

An Absorption Cycle Air Source Heat Pump – in cooling (chilling) operation
Unlike vapour compression heat pumps that use environmentally harmful HFC refrigerants as their working fluid, some of which are over 1,000 times stronger greenhouse gases than CO2, absorption machines use one of two environmentally benign working fluid / absorbent “pairs”; ammonia and water or water and lithium bromide.

All of the Robur range of gas-fired heat pumps and chillers use the ammonia/water “pair” while the larger Yazaki range of chillers use the water/lithium bromide “pair”.
When it comes to measuring the efficiency of heat pumps and chillers/air conditioners it is difficult to compare electric machines with gas, hot water or steam driven devices.

The efficiency of an electric chiller or heat pump is defined as its COP

COP = Heat Output or Cooling Capacity (kW)
             Electric power used to drive the heat pump (kW)

The efficiency of a heat driven absorption chiller or heat pump is
defined as its Heat Utilisation Efficiency

HUE = Heat Output or Cooling Capacity (kW)
             Heat used to drive the heat pump (kW)

(or in the case of gas-fired units the GAS utilisation efficiency - GUE)

The COP and GUE are not comparable….. As gas is a PRIMARY fuel while electricity is a SECONDARY fuel (more often than not generated by burning gas !)

The ONLY way that the two technologies can be compared on an efficiency and environmental basis is to reduce the electric COP to a Primary Fuel Ratio by taking into account the efficiency of power generation and transmission – which in the UK is around 32%

PFR (elec heat pump) = COP * generation & transmission efficiency
                                         = COP * 0.32

PFR (gas heat pump) = GUE

Also – when comparing the two heat pump technologies – it is important to ensure that SEASONAL operating efficiencies and COPs are used and NOT just simply single design point data.