CPD Programme
Air Temp (˚C)
7 7 7
-7
Water Temp (˚C)
35 50 65 50
Heat Output (kW)
41.2 38.3 30.0 31.5
Fuel Input (m3
2.62 2.62 2.62 2.62
The commercial boilers would operate in the event that the thermal load exceeds the heat pump output. Alternatively the heat pumps could be sized for the lower grade heat loads (underfloor heating and hot water pre-heat) with the commercial boilers serving the higher temperature radiator circuits or main hot water calorifier. An example application of a gas absorption heat pump supplying heating and domestic hot water is shown in Figure 4.
Gas-absorption heat pump performance Gas-absorption heat pumps deliver a high level of fuel efficiency during operation, compared to traditional heating boiler plant, if used with lower load temperatures of around 35C. Higher efficiencies can still be achieved at load temperatures of up to 50C. During operation, the refrigerant cycle uses natural gas (or LPG or, potentially, biogas) assisted by the energy in the air, rather than electricity to provide heat. As the carbon emission factor for gas is lower than grid- supplied electricity,(grid supplied electricity = 0.544 kgCO2/kWh and natural gas = 0.184 kgCO2/kWh [4],) improved emission reduction benefits are offered, compared with traditional electric heat pumps. Ongoing heat- pump operating costs are also lower. The starting current of a gas absorption
heat pump is considerably lower compared with more conventional ground and air- source heat pumps, reducing the reliance on local electricity supply infrastructure. There is no compressor used within the refrigeration cycle – the solution pump (see Figure 1) used within the system to raise the solution pressure only consumes a small quantity of electricity; for example, for a 41kW gas-absorption heat pump, the electricity requirements for the solution pump is only around 1.1kW. Overall gas-absorption heat pump performance is measured in efficiency terms, and not simply the traditional coefficient of performance (COP). Table 1 outlines the performance of an example unit when used with different ambient air temperatures and water flow temperatures.
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/hour)
Gross CV of Gas (MJ/m3
)
38.5 38.5 38.5 38.5
Table 1: Efficiency table for a 41kW-rated gas-absorption heat pump
Conclusion Gas-absorption heat pumps offer an innovative, high-efficiency solution for low- grade heating and hot water generation solutions, whether used on their own or in conjunction with commercial boilers; and their use can contribute to reduced carbon emissions and improved building environmental ratings. With the increasing pressure being applied to the UK heating and ventilating sector to deliver solutions with higher efficiencies and a lower carbon footprint, gas-absorption heat pumps have the potential to provide part of the solution. © Tim Dwyer and Yan Evans
Figure 3: A commercial gas-absorption heat pump By combining the energy available from
natural gas with that extracted from the surrounding ambient air, gas-absorption heat pumps offer extremely high fuel efficiencies. For example, with an ambient air temperature of 7C and a water load temperature of 35C, a heat pump efficiency of around 143% can be achieved. This is the efficiency using the gross calorific value of natural gas and after deduction of the parasitic loss (ie the solution pump, fans and controls energy use). If applied in the appropriate manner, they have the ability to deliver fuel efficiencies that attain the A+++ level, which is likely to be defined under the EuP Directive.
References: 1. The Leadership in Energy and Environmental Design (LEED) 2009 for New Construction and Major Renovations Rating System.
www.usgbc.org/ShowFile. aspx?DocumentID=7244 Accessed 24/8/2010
2. TM13 2002: Minimising the Risk of Legionnaire’s Disease. CIBSE, 2002
3. ACoP L8 – Legionnaire’s disease: the control of legionella bacteria in water systems. HSE, 2000
4. Conversion factors – Energy and carbon conversions – 2009 update. Carbon Trust, 2010
Outdoor Weather Sensor
Temperature Control Regulator Room Thermostats
28.0 28.0 28.0 28.0
Gross Fuel Input (kW)
Efficiency Gross CV of Fuel
Electricity Consumption (kWe)
147.1% 1.09 136.8% 1.09 107.1% 1.09 112.5%
1.09
Net Efficiency Gross CV of Fuel
143.3% 132.9% 103.3% 108.6%
EuP
Directive Benchmark
>132% >132%
A+++ A+++
>100% A+ >100% A+
Rating
System Pumps
Expansion Vessel
Gas fired heat pump
Primary Pump
Indirect Domestic Hot Water Cylinder
Direct Buffer Cylinder
Figure 4: Schematic of example system October 2010 CIBSE Journal 69
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