Active Solar Strategies

Before looking at each technology, here are a few practical considerations that apply across all three.

Most domestic roof mounted installations are classed as permitted development and don’t need planning permission, as long as the panels project no more than 200mm from the roof and sit below the ridge. The main exceptions are listed buildings, which always require Listed Building Consent, also conservation areas, AONBs and World Heritage Sites or anywhere an Article 4 Direction applies, where planning permission is often needed. It is always worth checking with the local authority first before specifying.

All installations must also comply with the Building Regulations, covering structural loading, electrical safety and energy efficiency.

Finally, whichever technology you choose, using an MCS-certified installer is crucial as it is usually a condition for accessing grants and incentives.

Solar Thermal

Solar thermal is one of the most established low and zero carbon technologies. It predates the photovoltaic panels we are so used to seeing on roofs today and is used in both new builds and retrofit dwellings alike.

A solar thermal system collects energy from the sun and uses it to heat domestic hot water. The system will usually need a supplementary heating source (boiler or immersion heater) as it is unable to provide a household’s full hot water demand year-round. The systems are normally sized to meet between 50-70% of annual hot water demand in the UK. The Energy Saving Trust puts the seasonal split at roughly 90% of hot water in summer, dropping to about 25% in winter.

There are two main types of solar thermal systems: the flat plate collector and the evacuated (vacuum) tube collector.

Flat plate collector
The flat plate collector consists of a network of dark, heat absorbing pipes set in an insulated bed, within a box that is glazed on top. They are best suited to situations where the demand is for lower water temperatures between 30˚ and 70˚. In the UK they are often the standard choice because they are simpler, cheaper and less prone to summer overheating.

Evacuated (vacuum) tube collector
The vacuum tube collector can be more efficient than the flat plate collector especially in cold or low light conditions. This is because the vacuum around each tube minimises heat loss. As a result, they can achieve higher operating temperatures and generally perform better during winter months. The collector absorbs solar energy to heat a fluid within the tubes, which then transfers heat to the domestic hot water system through a heat exchanger. These do cost more and have a greater risk of overheating in summer.

Both collector types are widely used in the UK. The flat plate collector is often described as the standard, lower-cost option while the evacuated tube collector is favoured when efficiency and winter performance are priorities.

Design considerations

• Solar thermal collectors need to be positioned on a south facing incline. It is said that a solar thermal system can be effective up to 45˚ east or west of south.
• The collector needs to be at a tilt of around 35˚, for optimum performance in the UK.
• Shading must be avoided to reduce any losses in performance of the system.
• The hot water tank is usually positioned in the roof space or an airing cupboard which needs to be accounted for both in terms of access and structure.
• The storage cylinder needs to allow for about 40-50 litres per person, with a minimum of 50-60 litres of storage per m2 of collector. A typical family of four would need a cylinder of around 200-300 litres.
• A twin-coil (solar) cylinder is required. One coil for the solar circuit and one for the back-up boiler/immersion.
• The cylinder should have high-performance insulation to minimise heat loss.
• The primary circuit needs freeze protection (typically a glycol/water antifreeze mix) for UK winters.
• Overheating and stagnation in summer must be managed by design, especially with evacuated tubes.
• The system works well combined with underfloor heating or similar lower temperature systems.
• During the winter around 25% of hot water needs will be met, while in the summer, roughly 90% hot water needs should be met.

Advantages

• No fuel cost for the solar energy collected.
• Reliable technology with relatively few components so maintenance is low.
• High solar-to-heat conversion per m2 compared with using PV to heat water.
• Reduces gas/electricity use for hot water and lowers carbon emissions.

Disadvantages

• Provides hot water only and does not generate electricity.
• Intermittent and seasonal supply of sun with little useful output on dull winter days.
• Requires a back-up heat source and a compatible twin-coil cylinder.
• Needs roof space, structural allowance and access for the cylinder.
• Overheating/stagnation and freeze protection add design complexity.
• Solar thermal needs a stored cylinder, so combi-only systems generally are not compatible with it.
• Payback can be long relative to the falling cost of solar PV, which is why PV (or PV + a heat pump) is increasingly chosen instead.

Solar thermal currently has little dedicated financial support. A zero rate of VAT does apply to qualifying domestic solar thermal installations in Great Britain, from 1 April 2022 until 31 March 2027. However, to access support schemes and warranties the installations should be carried out by an MCS-certified installer.

Solar Photovoltaic (PV)

Photovoltaic panels generate electricity by converting sunlight into direct current (DC) electricity, which is then converted to alternating current (AC) by an inverter for household use. With the cost of PV technology decreasing, and its performance improving, this renewable technology is becoming increasingly popular in UK homes.

Traditionally PV arrays were installed on existing buildings, usually fixed onto an existing roof. Now, with the advancement of technologies to include integrated roof tiles, glass curtain walling and other roof glass panels, we see PV being specified on a wider more varied selection of new build projects.

The efficiency of a photovoltaic panel is about 20-22%, meaning that percentage of the sun’s energy is converted into electricity. Panel manufacturer datasheets will give you the exact figures for a specific product. For a general idea, the Energy Saving Trust gives a typical domestic system to be around 3.5 kilowatt peak (kWp), covering between 10-20m2 of roof surface area. They also say between 6-12 panels would be used in this system. You can also model your own roof with their solar panel calculator.

The PV panels are categorised by the type of photovoltaic cell they use. There are three main types:

Monocrystalline panels
These comprise cells cut from a single silicon crystal, giving the highest efficiency and a uniform black appearance.

Polycrystalline (multicrystalline) panels
These have cells made from multiple silicon crystals, giving a blue, speckled look and lower efficiency.

Thin film panels
These panels use a thin semiconductor layer on a glass or flexible backing. They are cheaper and lighter but less efficient.

Apart from the cell type, PV also comes in different form factors. From conventional framed panels fixed to a roof, to building-integrated PV (BIPV) such as in-roof systems, solar roof tiles and PV glazing.

The efficiency of the photovoltaic will depend on many factors which all must be considered during the careful design process.

Design considerations

• Panel orientation is key to successfully achieving optimum performance (see table). Within 10˚ of south (up to 30˚ is acceptable) and panel angle between 30˚ and 40˚ delivers the best output.
• Any shading must be avoided which includes trees, other buildings, chimneys, parapets as these can significantly decrease performance.
• Access for maintenance and cleaning must be considered, on a flat roof a 1m walkway around the panels must be kept clear.
• Structural weight and wind loads must be considered as part of the design.
• PV requires good circulation around the panel to avoid overheating which will have a significant effect on generation. At least 150mm air circulation is required.
• A lighter roof/mounting surface helps to avoid overheating.

The following table shows the effect of tilt and orientation on energy output.

[Image adapted from The Environmental Design Pocketbook – Sofie Pelsmakers]

Advantages

• Any generated electricity that isn’t consumed can be fed to the national grid and paid for under the Smart Export Guarantee (SEG) – see below.
• Battery storage can store surplus generation for use later, improving self-consumption.
• Few moving parts.
• Can be integrated into the building fabric (in-roof systems, solar tiles, glazing).
• Panels set at different angles can achieve optimum efficiency at different times of the day or year.
• The fuel (sun) is free and there is no noise or pollution created from the PV panels.
• The systems have long service lifetimes (with panel warranties for 20-25+ years).

Disadvantages

• Intermittent and seasonal supply of sun.
• Difficulty storing the generated electricity.
• Expensive to purchase and install.
• Long payback time.
• Requires careful design and specialist (MCS-certified) installation to maximise performance.
• Must be installed away from shade or potential future shade.
• Requires regular cleaning to ensure maximum performance.
• Generates full output for only a small fraction of the year.

Under the Smart Export Guarantee (SEG), in force since 1 January 2020, larger licensed electricity suppliers must pay you for any electricity you export to the grid. SEG pays only for the units you export, not for everything you generate, and each supplier sets its own rate. So, it’s best to compare tariffs, as returns vary widely.

Just like the solar thermal system, zero rate of VAT also applies to qualifying domestic PV and battery installations. Low-income or qualifying households may access funded PV through schemes such as the ECO4. But this scheme is only available until 31 December 2026 and it will not have a successor supplier-obligation scheme, as the Warm Homes Plan is taking its place. In all instances, the installations should be MCS-certified to qualify for SEG and warranties.

Photovoltaic Thermal (PV-T)

PV-T or PVT are panels which generate both electricity and heating from a single module. A heat exchanger sits behind or beneath the PV cells and carries away their waste heat using a circulating fluid.

They have the potential to produce more energy than a standard PV by using the overheating of the panels to its advantage, drawing heat to be used as a heat source for the dwelling. This would be achieved using an air source heat pump.

PV-T collectors are usually classified by the fluid that carries the heat away, and by whether the front is glazed.

Liquid (water) PV-T
This circulates water or a water/glycol mix through a heat exchanger bonded to the back of the cells. It is the most common type for domestic use.

Air PV-T
This passes air across or behind the cells instead of a liquid. It is simpler, with no leak or freeze risk, but produces lower-grade heat. It is less common in UK homes.

A further distinction is glazed vs unglazed. A glazed (covered) collector has a glass cover and insulation, producing more heat but slightly less electricity. An unglazed (uncovered) collector is essentially a PV panel with a heat exchanger on its back, producing slightly more electricity and lower-grade heat which makes it a good match for heat pumps.

PV-T remains a relatively niche technology in UK homes, used where roof space is limited and both heat and power are wanted from the same area.

Design considerations

• Best suited to sites that want both hot water/heat and electricity but have limited roof area for two separate systems.
• Needs the same careful orientation, tilt and shading considerations as standard PV.
• Requires plumbing for the thermal circuit (cylinder, pump, controls, freeze protection) in addition to electrical works, so installation is more complex.
• Pairs well with a hot water store and/or a heat pump.

Advantages

• Produces electricity and useful heat from the same footprint.
• The cooling effect can improve PV electrical efficiency compared with a standard panel.
• Higher overall (combined) energy efficiency than separate PV and solar thermal of the same area.

Disadvantages

• Higher cost and complexity than standard PV or solar thermal alone.
• Comparatively new, less widely used technology with fewer installers.
• Thermal output is low-grade and seasonal, like solar thermal.

The PV (electrical) component can qualify under the Smart Export Guarantee (SEG) where the installation meets eligibility/MCS requirements. The zero rate of VAT applies to qualifying domestic installations.