Mixing technology for maximum efficiency

There is no question that the UK heating and ventilation sector is going to face some tough technical and engineering challenges over the next few years. Yan Evans, technical director of Andrews Water Heaters and Potterton Commercial, offers some innovative thoughts and methods on how to deliver ultra-low carbon heating and hot water solutions.

Changes to Part L2 of the building regulations are due in 2010 and in the same year the controversial EuP (Eco-Design of Energy Using Products) Directive will be introduced. These measures are already starting to apply pressure throughout the industry as increasingly stringent regulatory policies place demands on the design and supply chain to deliver high system efficiencies and low carbon buildings.

The EuP Directive is set to change the whole map of the residential and commercial heating and ventilation sector.  Following its introduction in 2010, key milestones in 2011 and 2013 will dictate minimum efficiencies for systems providing space heating and hot water.

In 2011 the minimum system efficiency will be 56% and in 2013 a significantly more challenging 96%.  This efficiency figure relates to the system rather than the appliance and, for space heating, is measured from fuel input to heat emission. It is therefore going to take some clever combinations of technologies to achieve the new requirements.

The best available commercial boilers and direct-fired water heaters can provide efficiencies of around 96% to 98% on gross calorific value of fuel.  Once the energy consumption and losses of other system components such as primary loop pumps, secondary return pumps, valves, hot water cylinders and pipework is taken into consideration, the efficiency of the entire system will inevitably fall short of the 96% required under the EuP Directive in 2013.

But delivering significant carbon savings that meet the minimum system efficiency just isn’t enough.  The EuP Directive efficiency scale starts at G moving through to A+++ at the very top.  This triple star rating equates to a minimum system efficiency of 120%.  It should be emphasised that this is not a net efficiency, but an efficiency based on gross calorific value of the fuel, that can be metered and measured.  So, with gas fired commercial boilers and water heaters at the heart of the systems and the best examples of these products currently available being utilised, how are the really high efficiencies going to be achieved while maximising reductions in carbon dioxide emissions?

Heat pumps

Heat pumps are being regarded very highly by the EU Commission in the formulation of the EuP Directive as one of the technologies to support the achievement of high system efficiencies.  This applies to ground source heat pumps, air source heat pumps and gas absorption heat pumps.

If applied and integrated in the appropriate manner ground source heat pumps can deliver a Coefficient of Performance (CoP) of up to 4.7.  This is based on a ground loop temperature of 0ºC and a heating load temperature of 35ºC. With this low grade load temperature necessary to deliver such a high CoP, the ideal applications would be under-floor heating and cold water pre-heat to direct-fired water heaters.  A CoP of 4.7 means that for every 1kWe of electricity consumed to power the ground source heat pump, it will produce 4.7kW of heat. 

Energy from incident solar rays is stored in the earth.  Ground source heat pumps require a ground loop to absorb this energy and sustain the refrigeration cycle. Thus for commercial applications where the ground loop could comprise a large number of horizontal trenches and deep bore holes, ground source heat pumps are better suited to new build properties rather than the retrofit market. 

On the other hand the absence of the need for a ground loop with air source heat pumps – energy present in the ambient air is used for refrigerant evaporation – allows the technology to be retrofitted into existing properties and should be considered as a viable option to help assist in improving the energy rating of our existing commercial building stock, be this public or private sector.

Air source heat pumps offer a lower CoP of around 3.2.  This would be based on an ambient air temperature of 2ºC and a hot water load temperature of 35ºC.  So, as with ground source heat pumps, applications requiring low grade heat are most suited to this technology.

Saving energy

There is no question that the energy conversion factor for air source and ground source heat pumps is admirable. When coupled with the fact that the energy used to support the refrigeration cycle (air or ground) is from a natural source, both forms of heat pumps can be regarded as a renewable technology.

However, the fact remains that these heat pumps use electricity as the primary source of energy. If this electricity is grid supplied then this has a significant impact on the environmental benefits due to the carbon intensity of centralised power generation.  In the UK the carbon dioxide emission factor for grid supplied electricity is 0.43 kgCO2/kWh, based on the current generation mix. This is considerably higher than the factor for natural gas which is 0.193 kgCO2/kWh.

If we consider an air source heat pump with a heat output of say 20kW, then with a CoP of 3.2, the heat pump would require around 6.2 kWe of electricity to power the refrigeration compressor, fan and controls. If this electricity was to be either wholly or partly supplied by the electricity generated by a micro-CHP (Combined Heat & Power) unit installed on site, then this would effectively displace the grid supplied electricity powering the heat pump, by natural gas fuelling the micro-CHP unit.

Using CHP

The diagram in Figure 1 is a simplistic representation of how an embedded generator, here in the form of the micro-CHP unit, can be used in conjunction with a heat pump to reduce the overall carbon emissions. The Baxi-Senertec DACHS micro-CHP unit in this example is providing a contribution of 5.34kWe of 3-phase electricity towards the electrical energy requirements of the air source heat pump.  Here the heat pump requires less than 1kWe of electricity to be imported from the national grid.  This would have a profound effect on the carbon footprint of the solution. Considering the fuel input to the DACHS unit and the reduced grid supplied electricity for the heat pump, and the combined heat output of both products, the resultant overall efficiency is in the region of 140%.  This is well above the 120% required for the A+++ designation under the challenging EuP Directive.  Even after some of the system components consumed energy, and taking other losses into account, we would still expect the efficiency rating to be achieved.

However, this example does not show the entire picture.  The heat output of the micro-CHP unit would be used to satisfy the base heating load of the property in order to maximise annual running hours.  Peak heating demand periods would be supported by high efficiency condensing commercial boilers.  On the hot water aspect of the system the air source heat pump could be used to pre-heat the cold water into a direct-fired water heater raising the incoming cold mains from 10ºC to say 30ºC.  This would significantly reduce the natural gas fuel input to the primary hot water appliance, but at the same time maximise the performance of the heat pump whilst keeping the outlet temperature relatively low.

This same concept could be applied to a ground source heat pump or gas absorption heat pumps with varying output micro-CHP units being used to support the solution, chosen depending on the electrical load requirements of the heat pump.

Achieving the rating

n summary it is very likely that in the commercial sector a combination of conventional products such as commercial boilers and water heaters working in harmony with renewable technologies such as heat pumps, supported by micro-CHP units as a form of embedded generation, will be necessary to achieve the coveted A+++.  There is no doubt that there are other technology combinations that can be considered and product selection must be application driven. 

The thermal load needs to be present for the combination of technologies chosen, but there is no reason that, with the appropriate application and merging of solutions, ultra-low carbon heating and hot water solutions cannot become common place in our industry.