A marriage of technologies

Published:  08 September, 2008

Challenging and interesting times — Paul Hardy.

With legislation, Building Regulations and local-authority planning consent applying increasing pressure on the heating and ventilation sector to deliver commercial plant rooms with the lowest possible carbon footprint, Paul Hardy explains how the integration and harmonising of low- and zero-carbon (LZC) technologies with conventional products has the potential to achieve the ultimate solutions for space heating and hot-water generation.

In the commercial building-services sector we are beginning to experience designs for heating and hot-water systems that comprise a variety of heat sources. This is partly in response to legislation and Building Regulations, but mainly as a result of local-authority requirements for planning consent.

Design engineers are being placed under increasing pressure to deliver solutions where up to 15% of the energy must be derived from some form of renewable technology. In turn these engineers are looking to manufacturers to provide support in not only supplying products such as solar thermal solutions, heat pumps and combined heat and power (CHP) units, but in how all this equipment can be integrated together and also with condensing boilers and direct-fired water heaters.

A certain degree of caution is required when considering how to achieve a low-carbon plant room, as some of the technologies mentioned above work well together, whereas others can operate with some element of conflict if applied in the incorrect manner. Two technologies that work well together in the correct circumstances are ground-source heat pumps and combined-heat-and-power units.

The Earth acts as a huge thermal store of energy from the Sun. At a depth of 1 to 2 m the temperature of the ground is fairly constant at around 12°C.

Ground-source heat pumps harness this energy through the use of a ground loop which extracts heat to commence and sustain the refrigeration cycle within the heat pump. Through a process of changing the state of the refrigerant, heat is generated which can be used for space heating. The maximum efficiency, or Coefficient of Performance (COP), of the ground source heat pump is achieved when the heating medium into which the appliance is integrated is low grade, such as under-floor heating where the required water temperature is around 35°C. At this water temperature some of the best available ground-source heat pumps can achieve a COP of around 4.7.

This thermal energy would displace fossil fuel that would normally be used by a conventional boiler to produce heat. Although the prime source of energy for the heat pump is electricity, the high conversion rate of electrical energy into thermal energy delivers a significant reduction in carbon-dioxide emissions.

The mix of electricity generation in the UK gives carbon-dioxide emissions of 0.43 kg/kWh, according to DEFRA. If we consider a ground-source heat pump with an output of 21 kW and a COP of 4.7, operating for 3000 h a year and displacing natural gas into a condensing boiler with a gross seasonal efficiency of 96%, the reduction in carbon dioxide emissions would be in the region of 6300 kg a year.

In a commercial plant room it is likely that the ground-source heat pump would satisfy the base thermal load rather than meeting all the heat requirement. There may also be a combination of low-grade heat being required to serve underfloor heating and higher temperatures for radiators. This is where the combination of the ground-source heat pump and high-efficiency condensing boilers works well together. The lower-grade heat load is supplied by the heat pump to sustain a high COP, and the higher grade heat is supported by the boilers.

Despite the high COP delivered by the heat pump the fact remains that it is an electrical appliance, and electricity has a high carbon index compared to natural gas. However, if the electricity required by the heat pump were to be supplied by some form of micro-generation then the carbon footprint of the solution would be considerably reduced.

High-efficiency condensing boilers combined with technologies such as ground-source heat pumps and CHP have a valuable role to play in reducing carbon-dioxide emissions.

CHP is the simultaneous generation of electricity and heat at the point of use from a single appliance, and CHP products can offer significant reductions in carbon-dioxide emissions through the displacement of boiler fuel and electricity that would otherwise be imported from the National Grid.

At the core of the appliance is either an internal combustion engine or gas turbine. CHP units are heat-led devices, and the thermal output should be selected to match the base heating load of the property to maximise both annual running hours (typically 5000 to 6000) and the economic and environmental benefits.

Base electrical load should also be taken into consideration when sizing the CHP unit to minimise the spilling of electricity to the grid, as in the UK there is currently little or no financial incentive in doing so.

As a guide, though, the reduction in grid-supplied electricity and displaced boiler fuel of a CHP unit with an electrical output of around 5 kW and heat output of 12.5 kW operating for around 5000 h a year would reduce carbon-dioxide emissions by about 5000 kg.

On the assumption that the property has sufficient base thermal load to support the ground-source heat pump, with CHP unit outputs sustaining operation of both appliances, this combination could deliver an ultra-low carbon solution. Peak heat loads could be met with high-efficiency condensing boilers to complete the solution, with electricity required above the output of the CHP unit imported from the grid.

The suggestion above represents only one possible scenario, and there are inevitably many other technology combinations. As heating and hot water systems become more complicated, with a mixture of heat sources being utilised to deliver the desired reduction in carbon-dioxide emissions, the UK building -services industry has some challenging and interesting times ahead.

Paul Hardy is managing director of Andrews Water Heaters and Potterton Commercial.



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