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Solar thermal heating and hotwater heating systems

Solar thermal technology

For over fifty years solar thermal heating has been used effectively in the UK. The long-term target by the EU is to have 1 m2 of solar collector area per inhabitant. The majority of systems supply hot water heating for domestic buildings, although systems are also used for swimming pools, industrial, commercial and public sector buildings. In the domestic market, solar hot water heating systems can provide almost all of your hot water during the summer months and about 50% year round.

All solar thermal collectors operate under the well-known principle that radiation from the sun heats the surface of materials and this heating effect is most prominent on dark coloured surfaces. There are two common types of solar collector that are used to provide hot water and space heating in domestic and commercial situations: flat plate collectors and evacuated tube collectors. Over the normal domestic hot water operating temperatures, there is little difference between selective surface flat plate collectors and evacuated tube collectors, however, evacuated tube collectors perform marginally better overall. Despite this, a lower efficiency panel will simply require additional collector area to be installed to achieve the same output, and frequently collector choice will come down to the preferred aesthetics or roof space available. Generally, evacuated tube collectors offer the highest operating efficiency all year round and, due to their low weight and smaller dimensions, offer easier installation and take up less roof space per system. Flat plate collectors should be used when efficiency during autumn, winter and spring is not so important and in areas where a tough collector is preferred, e.g. in areas at risk of vandalism.

Collectors can be roof mounted, roof integrated (in new build or new roofing situations), mounted on flat roofs, or mounted on A-frames at any level. The optimum orientation is south facing with an angle of inclination of 35 degrees. However, any orientation from south east to south west with an angle of inclination between 5 and 50 degrees is fine and suffers less than a 10% reduction on the optimum annual energy collection. If this is not possible, panels can if laid flat if necessary or an east-west system could be installed.

Solar system configuration

The key point to note is that solar thermal systems are designed to offset and not replace the conventional heating supply. In most retrofit situations, this will involve upgrading your hot water cylinder to an efficient twin coil solar type. In the winter, the solar heat energy preheats the hot water, and in the summer, it will often fulfil cylinder heating demand through the day such that your secondary heat source is not required at all. In most instances, a boiler (gas, oil, solid fuel, or electric) fired heating and indirect hot water cylinder system, can be upgraded to incorporate solar by a straight cylinder change, and where possible, this is usually a simple and cost effective project. If you have a gas or oil combination boiler, then there are two or three installation alternatives depending on the type of combination boiler you have. Some newer models allow a pre-heated supply from a solar cylinder that then bypasses the boiler when up to temperature. In other systems, where this is not possible, there are various ways of installing a solar hot water scheme that is compatible with the existing system which we would be happy to discuss with you.

In new build situations, all that is required is the specification of a suitable solar cylinder and sufficient space made available for the pumping station and controller in the cylinder cupboard or plant room.

Frequently, customers now decide to install an un-vented hot water cylinder or a thermal store cylinder. Both provide mains pressure hot water to your appliances. A thermal store cylinder has the advantage of storing the thermal energy from any heat source whether it is solar thermal, solid fuel boiler, conventional boiler, or a mixture of these. This heat energy is then used to deliver heat to the space heating system or to a heat exchanger through which the hot water supply passes. When used in conjunction with these thermal store cylinders, solar thermal systems have the ability to part load both your heating and hot water requirements. If the project is a new build, then careful consideration should be given to an under-floor heating system as the low temperatures this runs on saves energy and better utilises this solar gain.

In larger domestic and commercial situations, solar thermal energy from banks of collectors can be used to pre-heat a water store that is subsequently brought up to temperature in a secondary store or passed through an instantaneous water heater.

Typical Domestic Solar Installation Flat Plate and Evacuated Tube Collectors

We realise that the options are often overwhelming so Hendra Renewable Energy will advise, design and install a tailored system to suit your requirements and your budget. We insist on conducting a thorough survey of your house or premises free of charge to make sure that your system is correctly specified. From initial design advice to installation and maintenance, we will support your project throughout its life.

Costs and energy savings

As with any system it is very difficult to provide guideline costs as every project will have its own intricacies and special requirements. Costs range from £2,600 for basic small domestic installations to £20,000 for very large domestic, swimming pool heating, or commercial installations. Typically a larger domestic installation for a three bedroom house with a daily hot water consumption of 160litres at 55 degrees Celsius will comprise the following:

Four evacuated tube collectors with a total collector area of 5.16m2 (total active collector area = 3.22m2). Each collector measures 1.7m x 0.8m x 0.1m with 16 evacuated tubes and weighs 19 kg when empty.
Roof brackets and lead slate(s).
Resol solar controller and temperature sensors.
Solar pipe-work, expansion vessel, fittings, heat resistant insulation, and pump unit.
Thermostatic mixing valve – required to ensure hot water is discharged from your hot water storage cylinder at a safe temperature.
Solar fluid (provides temperature resistance, protection, and anti-freeze).
300 litre un-vented twin coil solar cylinder and ancillary pipe-work and fittings – provides mains pressure hot water.
Installation and commissioning.

The annual average solar gain expected is approximately 53% which will save approximately 600kg of CO2 when offsetting an oil boiler. The installed cost in such a case may be expected to be approximately £5,000. A system such as this should generate over 2kW of heat energy on a clear summers day. A reduction in specification – reducing the size of the cylinder, the number of panels and using a standard vented cylinder replacement will significantly reduce this cost.

Cost and performance data for a typical domestic solar water heating system

Typical system area	3-4 m2
Typical system price Retrofit (excl VAT)	£2,500-£6,000
Annual pump running costs	£6/year
Installation time	1-3 days
Assumed lifetime	25 years
Annual Output	1,000 – 1,500 kWh

Larger solar thermal systems for public/commercial buildings for the provision of hot water and, or space heating typically provide ~ 3000 kWh/yr in the UK.

A 25 yr lifetime can be expected from a typical solar thermal installation.

The payback varies between 8 and 20 years depending on the displaced heating system and the efficiency of the system installed. New build domestic and community / small commercial schemes tend to have a pay back in the region of 10-12 years and retrofit installations a payback in the region of 15 years (this assumes grant funding is received and electric heating is displaced). However, this is based on today’s energy prices and does not include for the potential for a price hike on conventional fuels or indeed decreasing availability of these fuels in the future. Solar systems will reduce the influence of such factors on your living conditions.

The cheapest system, a one or two panel installation with a vented twin coil cylinder replacement for a small domestic premise may be expected to cost £2,600 installed. Costs then increase with the type of cylinder specified, and as property size and hot water demand increase, where additional panels are required to maintain an economic solar gain. Un-vented and thermal store cylinders, provide great versatility in system design and provide mains pressure hot water but come at additional cost. Where a cylinder replacement may not be possible a separate pre-heat cylinder may be the best option, supplying the original cylinder. This is commonly the approach in commercial applications such as on farms and in hotels.

The costs quoted are all exclusive of VAT which is charged at 5% on all renewable technologies including all solar system components and at 17.5% on all other heating and hot water elements.

Grant funding
The grant available for householders is a maximum £400 regardless of size, subject to an overall 30% limit (exclusive of VAT). Grants available to private businesses, public sector organisations, community and charitable organisations, range from 30% to 50% up to a maximum, depending on the project and status. Please see http://www.lowcarbonbuildings.org.uk/about/ for further information.
Note householders must undertake a number of energy efficiency measures before applying for low carbon buildings grant.

Our engineers are fully trained and approved, enabling customers to apply for government grants towards the cost of the installation. Additionally, every aspect of all our installations will be certified.

Is my property suitable?
Preferably you will need a minimum of 2-4m2 of clear southeast to southwest facing roof receiving direct sunlight for the main part of the day. You'll also need space to locate an additional water cylinder if required.

Warranty
Solar hot water systems generally come with a 10-year manufacturer warranty and require very little maintenance. In addition to this, Hendra Renewable Energy will provide a one year warranty on all workmanship.

We believe that sustainable heating systems incorporating a renewable energy source will become a valuable investment to any home or premises, and could be of further financial benefit when the new energy efficiency ratings are introduced.

Planning issues

Planning permission is not normally required if the panels do not project significantly beyond the roof slope. However, exceptions may apply for systems on listed buildings, in conservation areas or areas of outstanding natural beauty. Consult your local planning authority if you are unsure.

For more information on grants and how solar systems work, please see:
http://www.greenenergy.org.uk/sta/ and
http://www.lowcarbonbuildings.org.uk/micro/solartherm/