Solar thermal panels use sunlight to generate heat for hot water. Solar thermal panels can be roof-mounted, free-standing, or integrated into the building fabric.

Requires: A roof that is not north facing (in the UK or at similar latitudes) or suitable ground area, Insulation.

Solar water heating or solar thermal technology harnesses the sun’s energy to heat water via solar thermal panels, also called collectors.

Although solar water heating performs best in direct sunlight, they still heat water on cloudy days or when the sun is low. The amount of hot water that they produce over the course of a day is variable, depending on seasonality and weather conditions.  Larger systems are typically more cost-effective. Solar water heating will not heat all the water required for a building but is usually designed to meet 50-60% of the water heating load over the course of the year, saving money and abating carbon dioxide (CO₂) emissions.

Any additional heating of water needed, for instance on a cloudy winter’s day, is provided by a conventional water heater. Bypass piping and valves allow the conventional system to provide hot water if the solar heating system is, for example, undergoing maintenance (1).

Solar water heating systems are composed of solar thermal panels or collectors, absorber fluid, thermal storage, an auxiliary conventional water heater, and, for active systems, controls. In general, 40-80 litres of water are heated per square metre of collector surface per day (2).

Solar water heating can be:

• Installed onto roofs. Solar tiles and slates designed to blend in with the aesthetics of a building are also available but cost more.
• Incorporated into roofs as in roof solar thermal. Like solar tiles this is more aesthetic but also more expensive.
• Installed on purpose-built auxiliary structures such as a canopy over car parks.
• Free-standing on the ground. (3)

A key consideration is that efficiency will be maximum when collectors are as close as possible to the hot water tank. Some systems can track the sun across the sky, increasing efficiency, but adding cost and complexity to the system. (1)

Solar water heating is especially suitable when:

• The cost of fuel for water heating is high, e.g., if the building is currently using electricity or LPG (liquid petroleum gas) fuel for water heating.
• The building is occupied in summer when solar water heating is most efficient.
• Water heating is required each day.
• Energy security is important (solar water heating works in conjunction with a conventional water heater providing an extra energy source).
• The climate is sunny. (1)

Mid-temperature solar water heating systems produce water 10-50°C hotter than air temperature. These systems are discussed below. The collectors utilised by these systems are usually flat plates, but evacuated tubes are also used. (1)

Flat plates collectors are typically made of copper or aluminium, coated with a black selective heat coating such as nickel to reduce losses through radiant heat. Insulation is typically provided by low-iron cover glass and either fibreglass or polyisocyanurate (1). Polymer flat plates are another alternative sometimes used. This design has the benefit of being less prone to freezing, thus removing the need for a heat exchanger, and increasing efficiency (4).

Evacuated tube collectors are made up of rows of cylindrical glass tubes with a core of copper pipes. This design yields very high levels of thermal insulation and helps concentrate sunlight. Tubes can be glass-glass, with two layers of glass fused together at both ends, for a more reliable vacuum but less light reaching the copper pipes and more corrosion. Alternatively, tubes can be glass-metal, for a less reliable vacuum but more light reaching the copper pipes and reduced corrosion. (3)

Compared to evacuated tubes, flat plates are cheaper to buy, longer-lasting (over 25 years to over 20), and more efficient at lower water temperatures. However, they are heavier and less modular, so more costly to install and replace and less efficient for heating water to higher temperatures. (1, 3)

In indirect solar water heating systems, sunlight hits a fluid such as propylene glycol in the collector. The fluid flows to a heat exchanger and the heat exchanged is used to warm water. In the UK, indirect systems may be favourable as they provide protection from water freezing in the collectors or pipes.

In direct solar water heating systems, the sun shines on water in the collectors which then flows into a hot water tank. To protect direct systems from freezing, one strategy is to drain the water from the collectors back into a drain-back tank. This configuration has an added advantage of protecting the system from excessive temperatures, if hot water consumption is reduced due to a change in use patterns but solar water heating continues. (1)

How and when solar heated water reaches the hot water tank is dependent on whether the solar water heating system is passive or active. Passive systems rely on convection currents to circulate the heated water. The hot water storage tank in such systems is either above the collectors (thermosiphon) or inside the collectors (integrated-collector-storage). (1)

Active systems have temperature difference controls for pumping water from storage to the hot water tank. The difference in temperature between the collector outlet and the bottom of the storage tank is known as ΔT. When the temperature difference increases above a set amount, the controls start the pump. Conversely, when the temperature difference falls below a set amount, the controller stops the pumps. The controls also have a high-limit function to turn pumps off if the temperature in the storage tank exceeds another value. (1)

As temperature difference controls are simple and low-cost, it is advised to keep controls independent of any wider Building Energy Management System, but some indication of system performance such as a preheat tank temperature sensor should still be included in any building control system. (1)

Following similar principles to solar panels used for Photovoltaics, in the UK solar water heating collectors should face south, tilted 30-40° from horizontal. East or west-facing roofs still warrant consideration, but north-facing roofs are unsuitable with present technologies and financial incentives. When siting collectors consider obstacles such as buildings, trees and chimneys that may provide shade thus reducing efficiency. Solar water heating collectors are lighter than photovoltaic solar panels so roof strength is not as great a consideration, however more attention needs to be given to anchoring collectors from the wind. (2)

The collectors are cleaned by rain if their tilt angle is greater than 15°. If dust, sand, or particulates settle on the collectors they may need to be wiped, as their efficiency is greatly reduced when dirty. For this reason, ground mounted collectors will need cleaning more regularly.14 Annual checks should be made on pump fluid pressure and pH levels, and any antifreeze replaced. Approximately every 10 years any heat transfer fluid will need replacing. (3, 5)

Solar water heating is generally classed as a permitted development, meaning planning permission is not required, however this should be checked on a case-by-case basis. (4)

(1) The Renewable Energy Hub UK, Information on Solar Thermal Systems. Available from: https://www.renewableenergyhub.co.uk/main/solar-thermal-information/ [Accessed 26^th January 2021].

(2) A. Walker, Whole Building Design Guide, Solar Water Heating. Available from: https://www.wbdg.org/resources/solar-water-heating [Accessed 26^th January 2021].

(3) The Renewable Energy Hub UK, Information on Solar Thermal Systems, Mounting Methods Solar Thermal Systems. Available from: https://www.renewableenergyhub.co.uk/main/solar-thermal-information/solar-thermal-panel-mounting-methods/ [Accessed 26^th January 2021].

(4) The Renewable Energy Hub UK, Information on Solar Thermal Systems, The Different Types of Solar Thermal Panel Collectors. Available from: https://www.renewableenergyhub.co.uk/main/solar-thermal-information/the-different-types-of-solar-thermal-panel-collectors/ [Accessed 26^th January 2021].

(5) The Renewable Energy Hub UK, Information on Solar Thermal Systems, Solar Thermal System Lifespan, Maintenance and Warranties. Available from: https://www.renewableenergyhub.co.uk/main/solar-thermal-information/solar-thermal-system-lifespan-maintenance-and-warranties/ [Accessed 26^th January 2021].