Solar Thermal Panels Explained

Harnessing the sun's energy could lower your heating bills considerably. Here's what you need to know when specifying a solar hot water array
by Chris Bates
18th November 2015

On average, UK households devote about 25%-30% of their annual heating spend on water for baths, washing up and similar activities. Solar thermal panels allow you to cut down on this portion of your consumption – and save on energy bills – by harvesting the sun’s warmth to generate domestic hot water (DHW). The general rule is that a well-specified setup can deliver around 60% of a family’s annual requirement.

How solar hot water works

There are two main configurations of panel to choose from: evacuated tubes or flat plates. Both of these products operate on the same basic principle of absorbing warmth via a heat transfer liquid, which is usually a mix of water and antifreeze.

This solution is circulated through copper pipes in the collector panels and pumped into a heat exchange coil that’s integrated into your home’s hot water cylinder – where the energy is transmitted into your DHW supply.

5 things to consider

  • Only your first solar installation will count as permitted development, so you’ll need to seek full planning permission for any panels added at a later date.
  • You’ll need a backup option to step in during periods when the collectors aren’t producing enough hot water. In most cases this will be either your boiler or an electric immersion.
  • Even during the colder months, the panels can still contribute to reduced energy bills by providing an element of pre-heating, which will reduce the load on the primary heat source.
  • Most conventional boilers and renewable setups are compatible with solar hot water. The main exception is combi boilers, as these systems don’t tend to include the required storage cylinder.
  • Solar panels should maintain good operation for around a quarter of a century and require very little maintenance. The anti-freeze mixture may need replacing every two to three years, while the circulation pump should last at least a decade.

Flat plate versions are popular for their slim profile, which means they can be fitted flush with the tiles if you’re installing panels on your roof. They’re a robust, cost-effective choice and usually come as pre-assembled kits. While this means they’re quick and easy to install, they’re also heavy – so you’ll typically need a two or three-person strong team on site. They’re fairly low-maintenance and – should they receive minor damage – easy to repair.

Evacuated tubes are widely considered to be the more efficient option. With this setup, individual collectors are sheathed in twin-wall glass tubes and sit proud of the panel. These pipes are under a vacuum, which enables better heat retention. As a result, evacuated systems can provide better output and are more effective in cloudy conditions (although both systems can still perform well even during overcast periods).

The main downside is that they’re pricier than flat plates. They’re also more prone to breakage – but their modular design means that broken tubes can be replaced on a one-off basis, rather than requiring you to have the entire panel swapped out for a new unit.

Planning & design

In the vast majority of cases, fitting solar thermal panels is considered permitted development (PD), which means you don’t have to apply for specific planning consent. There are some limitations, however: the PD rules don’t apply to listed buildings and the panels must not project more than 200mm above the roof slope. For the full list of requirements, see www.planningportal.gov.uk.

If you want to get cashback through the government’s Renewable Heat Incentive (RHI), your system should be professionally designed by an engineer registered with the Microgeneration Certification Scheme (MCS). Their aim will be to maximise delivery of your hot water requirements throughout the year. This should be eminently achievable for most homes, as only a relatively small area of roof is required to deliver good performance.

As a rough guide, you’ll need between 1m2 and 1.5m2 of panel per household member. Technically, solar thermal panels can be used for space heating, too – but this isn’t likely to be an efficient use of the panels and will rule you out of accessing the RHI, which at the time of writing pays 19.51 p/kWh only on MCS-registered installations that output to hot water.

The ideal scenario is to fit the panels on the main building’s roof, as this tends to be the most straightforward way to avoid shading issues that could cripple efficiency. But they can also be ground or wall-mounted.

A southerly orientation is generally preferred – although the setup will still achieve at least 70% of its optimum performance anywhere south of east or west. You can choose between in-roof systems, where the tiles are removed and the panels sunk in to achieve a flush finish (only available for flat plate collectors); or on-roof versions where the array is mounted on brackets attached to the rafters.

Solar thermal costs

Prices have fallen slightly since the introduction of the RHI back in April 2014, and today you can expect to pay from around £2,800 for a solar setup suitable for a household of two or three people.

According to research by the Energy Saving Trust, a typical setup of this size supplying a well-insulated home would net annual bill savings of around £65 per year compared to a standard gas boiler. On top of this, you can expect yearly RHI payments of approximately £195 (rising to as much as £470 for a six-person household).

Currently, if you choose a supply-and-fit system from a professional installer it will be subject to a reduced rate of VAT (5% rather than the standard 20%). However, a recent European Court of Justice ruling could see this discount abolished in the near future – and in any case, it doesn’t apply to DIY setups.

Photo:  Celtic Renewable Energy thermal panels

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