Taking a real shine to it- Solar hot-water heating is the solution of choice in helping deliver low-carbon-footprint buildings, according to Yan Evans, technical director at Andrews Water Heaters and Potterton Commercial.
Building regulations, legislation, environmental policies - and now planning consent - continue to drive end users, architects and design engineers in the UK to consider seriously the use of low-carbon and renewable solutions in residential and commercial properties.

Already, a number of local authorities have introduced
planning-consent requirements insisting that at least 15% of the energy required for a new-build property is derived from some form of renewable technology. A solar-thermal solution is seen as one of the simpler and most cost- effective ways of complying and reducing carbon emissions.

Good climate

The sun is the most abundant source of energy available to our planet. Solar-thermal solutions harness this energy for heating hot water. Using solar energy in this manner is a concept that has been available in the UK, in a variety of guises, since the mid-1970s.

Despite preconceptions - the weather conditions, and the location of the UK relative to the equator - the UK offers a good climate for solar hot-water heating solutions. The UK, in fact, experiences around 60% of the solar energy received at the equator. Only 55% of the light from the sun is visible, the balance being ultraviolet and infrared.

Furthermore, only about 25% of sunlight is direct, the balance being diffused, for example light
bouncing off clouds, buildings and the ground. All these factors must, therefore, be taken into consideration when designing products to collect solar energy.

In the UK the average annual available solar irradiation varies between 1,200kWh/m2 on the south coast, and 900kWh/m2 in Scotland.

The concept of solar hot-water heating is relatively simple. Roof- mounted solar collectors - with high transmission and absorption efficiencies - capture energy from incident solar irradiation, passing the heat into a transfer fluid.

This heat-transfer fluid is usually a mixture of water and glycol, to prevent freezing during periods of low outdoor air temperatures. The fluid is pumped through a coil located in the lower section of an unvented indirect cylinder. In so doing, the stored water is heated. Solar-thermal solutions are normally used for heating domestic hot water. During the warmer summer periods, when maximum solar irradiation is available, there is little or no need for space heating.

Seasonal variations

A well-designed solar-thermal system should be able to satisfy, on average, around 30% to 40% of the annual hot-water load. This value is known as the Solar Fraction (SF).

Because of the seasonal variations in available solar irradiation and weather conditions in the UK, during the colder and darker winter months the solar-thermal system may only be able to meet about 20% of the hot water demand. The balance would be supplied by the primary heating appliance - either a heating boiler or direct-fired water heater. In the summer months, during periods of high available solar irradiation, the SF can increase to close to 100%, depending on the hot water demand profile. During these periods, the solar energy that is absorbed by the collectors and transferred into the hot water can negate the need for any energy at all being provided by the primary heating appliance. This can have a significant impact in reducing CO2 emissions.

To give an example: direct-fired storage water heaters require a
certain amount of energy, depending on their efficiency, to raise the incoming cold-water
supply at 10˚C (typically) to a water temperature of, say, 60˚C. This is to ensure the stored water is at a sufficiently high temperature to destroy the legionella bacteria.

A pre-heat cylinder, served by an array of solar collectors, could be used to raise the temperature of the inlet water to the heater. The latter would then require less energy, and therefore less fuel, to raise the water to the required 60˚C.

In the summer months, there may be sufficient solar irradiation over prolonged periods of the day, such that the water in the pre-heat cylinder is able to reach temperatures in the region of 80˚C. In such circumstances, depending on how the pre-heat cylinder and collector array have been selected, the solar energy could be sufficient to supply the required water temperature at the outlets.

For commercial boiler applications, the main principle of generating the energy and the use of an indirect cylinder are the same as for direct-fired water heaters. The difference is in the design of the cylinder, in that it has two indirect coils.

The lower coil is served by the solar-collector array, and the top coil is served by the heating
boilers. This is usually via a low-loss header. The use of the latter is necessary in order to prevent the boilers from switching off on over-temperature. This is because the boiler capacity sized for the building's space-heating requirement is usually much larger in output than the rating of the coil.

In the event that there is insufficient solar energy to heat the water to the set point of, say, 60˚C, the commercial boilers would provide the additional energy required to raise the water
temperature.

The control of the transfer of energy from the collector array, and the indirect cylinder, is
conducted in the same manner for both solutions. This is differential temperature control, via a sensor at the outlet of the solar-collector array, and a sensor located in the lower portion of the cylinder.

When the temperature differential is greater than about 7˚C, the control unit switches on the pump. This allows the energy captured within the solar collectors to be circulated and transferred into the water via the indirect cylinder coil. When the temperature differential is less than 30˚C, the pump is switched off.

Much debate

For commercial applications the issue of development of legionella bacteria in the solar cylinder is often the subject of much debate. This is because the water could be stored at temperatures at which the bacteria can develop - 20˚C to about 45˚C.

Pasteurisation can be conducted, through the use of a shunt circuit between the storage water heater and the pre-heat cylinder, and the use of a destratification pump on twin coil cylinders.
There are two main types of solar-thermal collectors currently being used in the UK commercial building services sector. These are glazed flat plate and evacuated tube collectors.

The construction of a glazed flat plate collector consists of a lightweight aluminium tray, or frame, which contains a layer of insulation to prevent heat loss via conduction through the rear of the collector. A series of copper pipes is laid within the insulation, which carry the heat-transfer fluid through the collector.

Convection losses

A thin copper absorber is ultrasonically welded to the copper pipes. The absorber has a selective
coating in order to maximise solar irradiation absorption.
Finally, the collector has a transparent glass cover with a low thermal expansion coefficient, such as borosilicate glass, and high transmission efficiency to minimise convection losses.

Transmission efficiencies are typically 91%, absorption efficiencies 95%, and emissions 5%. The maximum thermal efficiencies, taken as an average during the year, are about 78%.
The construction of an evacuated-tube collector is entirely different to that of a glazed flat plate collector, although some materials used are the same. Copper tubes carry the heat-
transfer fluid, a copper absorber has a selective coating, and the tubes are manufactured from glass with a low thermal-expansion coefficient.

Evacuated-tube collectors comprise a manifold, and a series of glass tubes - typically 20-30 -
connected in parallel. A vacuum is created within each tube during the manufacturing process.

This effectively acts as an insulator for the absorber and reduces convection losses, particularly during colder winter periods. Whereas transmission efficiencies, absorption efficiencies and emissions are comparable to those offered by glazed flat plate collectors, the thermal efficiency is higher as a result of the presence of the vacuum - with values being
typically 83%.

Whether glazed flat plate or evacuated tube collectors are used, the optimum angle of orientation is south facing. The optimum angle of inclination is between 30˚ and 45˚ from the horizontal. Direct- flow evacuated tubes, where the heat-transfer fluid is pumped through each tube, offer greater flexibility regarding positioning. They can be placed flat on the roof or vertically on a façade, with the ability to rotate each tube to optimise orientation and
inclination.

The application of solar-thermal systems is relatively simple. On new-build projects there is work done on the roof of the building, and it is during this time that the solar collectors can be installed. Beyond the collectors, and in the plant room, the work is predominantly hydraulic. It can be carried out by the mechanical contractor tasked with installing the remainder of the equipment.

Energy captured

As solar-thermal systems generate heat, and not electricity, the installation is not complicated. There are no issues of parallel operation and exporting of electricity to grid, as is the case with technologies such a CHP photovoltaic panels and wind turbines. There is no ground work, as required with ground-source heat pumps, to recover heat. Also, the presence of solar collectors on the roof is a visible indication that the end user has taken positive action and invested in a technology that reduces the carbon footprint of the building.

Solar-thermal solutions offer an opportunity to significantly reduce CO2 emissions - approximately 100kg of CO2/m2 of collector array per annum, when compared with natural gas, which, in the UK, has a CO2 emission factor of 0.193 kg CO2/kWh (source: DTI), and a primary heating appliance with a gross thermal efficiency of 80% - contributing significantly to achieving the panacea of 15% energy derived from renewable technologies.

Domestic hot water is becoming the dominant thermal load as building air tightness and insulation levels improve and space-heating loads reduce. The area on which to focus the application of low- and zero-carbon technologies should be hot-water demand.

Solar-thermal systems can also offer an opportunity to reduce the carbon footprint of existing
properties as a retrofit solution.

In fact, there may well be an increase in the uptake of solar-thermal solutions in existing
buildings as a result of the introduction of Display Energy Certificates in public buildings.

The simplicity of installation and operation and the green benefit offered are believed to be the key reasons why so many commercial projects are adopting solar thermal-systems for delivering carbon reductions.