Yan Evans, technical director of Andrews Water Heaters and Potterton Commercial, explains why some heat-generating technologies work well together – and others do not.
A growing number of local authorities require at least 15% of the energy requirements of new-build property to be derived from some form of low- or zero-carbon technology. In response to this, design engineers are no longer looking to one appliance or solution to deliver this goal.

Daily, industry experts are being requested to review and comment on project hydraulic schematics that use different types of heat- generating products and solutions. The list includes solar-thermal solutions, ground-source heat pumps, air-source heat pumps, CHP products, biomass boilers, photo-voltaic collectors, and wind turbines.

This is not forgetting the more conventional technologies, such as high-efficiency condensing boilers and direct-fired water heaters.

Integrating technologies that generate electricity, such as photo-voltaic panels and wind turbines, is not such a big issue. But integrating solutions that generate heat is.

Photo-voltaic panels and wind turbines would generally be operating in parallel with the electricity-grid infrastructure. As such, even when connected to the local single- or three-phase distribution board within the property, the collector array or the wind turbine will effectively have access to an infinite busbar, through a connection via the main incoming supply.

There is no conflict between the technologies in gaining access to the electrical load, although the spilling, or exporting, of electricity to the grid needs careful consideration. This depends on the feed-in tariff available from the electricity company.

Plant room

But with products and solutions that generate heat, the chance of operational conflict because of heat load - whether space heating or hot water - is much greater. It is usually specific to the particular building being designed. This would not apply where properties are to be connected to a district heating scheme, where the heat source is a remote, centralised plant room.

CHP units are heat-led devices, and may not operate if the thermal load is insufficient, so the base thermal load must be determined when selecting the thermal and electrical output. This is if annual running hours are to be maximised, and the economic and environmental benefits of the unit fully realised. For many new-build properties, depending on the building use, and type of occupancy, the base thermal load will be limited to hot water during the summer months, with little or no demand for space heating.

Solar-thermal collectors deliver the maximum energy during the summer periods, when there are high levels of solar irradiation, prolonged daylight hours and higher ambient-air temperatures. Solar-thermal solutions are often sized for the daily hot-water demand, to maximise annual solar fraction - the percentage of the hot-water demand satisfied by the solar-thermal solution.

This sizing strategy would deliver a solar fraction of, typically, 40% to 50%. There is an expectation that the solar fraction during the winter months could be as low as, say, 20%, but in the summer months 100%.

This is with the solar-thermal solution satisfying the daily hot-water demand, negating the need for the use of any primary-heating appliances, such as boilers or direct-fired water heaters. Consequently, there would be no hot-water load to offer the CHP unit.

Economic benefits

In this scenario, the solar-thermal system would hold off the CHP unit, reducing the annual operating hours. This will have implications for the economic and environmental benefits of the installation.

As the CHP unit is displacing electricity, with a higher carbon intensity compared with natural gas, the operation of the appliance offering the greater CO2 benefit is being hindered. But the combination of solar-thermal solutions and CHP units can work in the correct application and suitable selected equipment outputs. If there is sufficient base thermal load to support the operation of the CHP unit and the solar collector array during the summer months, then the marriage of these two technologies can deliver significant CO2 reductions.

Annual reduction

As a guide, CHP units can deliver an annual reduction in CO2 emissions of about 0.98 tonnes per kilowatt electricity generated. This is compared with the current UK mix of centralised electricity generation - emission index of 0.00042 tonnes of CO2/kilowatt) - and heat generation by a commercial boiler with a seasonal efficiency of 80% fuelled by natural gas - emission index of 0.00018 tonnes of CO2/kilowatt.

For a well-designed solar-
thermal system CO2 emissions can be expected to be in the region of 0.098 tonnes per square metre of solar-collector array. This again compared with generated hot water using natural gas as a primary fuel.

Where a team of commercial boilers provides space heating, the solution for generating hot water - rather than using a calorifier - is to opt for direct-fired water heaters. The logical next step is to consider a renewable solution to work on each aspect of the system.

Using solar-thermal solutions to preheat the cold-water feed into a direct-fired water heater can result in a dramatic reduction in the amount of natural gas required to raise the water from a cold-water inlet of, say, 10˚C to a hot-water outlet of 60˚C.

Seasonal variations in the available irradiation mean that the contribution delivered by the solar-thermal solution in the summer months is far greater than in the winter.

Sizing the solar-thermal system, so that during the summer there is little or no primary heating appliance operation, could enable condensing storage water heaters to work in harmony with solar-thermal solutions. This would deliver an ultra-low-carbon hot-water generating solution.

Ground-source heat pumps harness the solar energy stored in the earth from incident solar rays. This is captured through a ground loop, comprising a series of plastic pipes buried in a trench, or installed within a deep bore. Ground-source heat pumps deliver the best performance when
serving low-grade heat, such as an underfloor heating array.

Peak demand

A ground-source heat pump could be installed to satisfy the base thermal load for building services space heating, with conventional high-efficiency condensing boilers providing supplementary heat during periods of peak demand. In an installation with an underfloor heating circuit, using a ground- source heat pump would maximise performance through water-outlet temperature control, enabling heating boilers to be used for the higher temperature radiator circuits.

This provides a low-carbon space-heating solution to compliment the low-carbon hot-water-generating solution offered through the use of direct-fired water heating and a solar-thermal cold-water pre-heat system. A further carbon footprint reduction is possible, depending on the application.

Ground-source heat pumps are effectively electrical appliances, but because energy in the ground is used to initiate and sustain their operation, they are classed as renewable technologies.

But, if there was sufficient base thermal load at the property to support the operation of, say, a ground-source heat pump and a small-scale CHP unit, then the electricity generated by the latter could be used to power the heat pump - dramatically reducing the overall carbon footprint of the plant room.

Similarly, it may be possible to use an air-source heat pump to pre-heat the cold water into a direct-fired water heater as an alternative to a solar-thermal solution. This is if, for example, there is limited roof space, or no south-facing roof, with a CHP unit being used to supply the
electricity demands of the heat pump. But caution is required, as the equipment needs careful
selection and control to optimise operation and performance.

Potentially, plant room design can offer significant reductions in CO2 emissions, and a hydraulic schematic that includes conventional fossil-fuelled appliances working with low- and zero-carbon technologies.

As well as equipment selection, the appropriate control of the operation and interface of a
variety of heat sources needs to be carefully thought out. The control strategy is the critical success factor for such installations, in order to ensure CO2 reductions are maximised, with no compromise in the provision of space heating and hot water.

System design

There is a need for more complex mechanical and electrical services on a project through the application of multiple heat sources. This means there will be more pressure on suppliers of conventional and low- and zero-carbon technologies for greater involvement in system design.

Andrews Water Heaters and Potterton Commercial are receiving more requests for a higher level of input into application engineering, a trend that is supported and encouraged.

To ensure project success they believe the equipment supplier should be regarded as part of the project team, and be seen as a solutions provider, with early participation in plant-room design.