SOLAR HOT WATER

Solar Thermal Hot Water Systems

The Sun and solar energy
Annually the UK receives solar radiation equivalent to the typical output of 1,000 power stations, and provides most of our renewable energies by driving wind, wave and water systems, and sustaining plant-life. Solar energy may be used more directly to provide domestic hot water. The average annual domestic hot water demand requires around 3,000 ? 4,000 KWh per household; with a total annual household energy demand of 26,000kWh on average.

Annually over 1000KWh of energy is received for each square metre of the British Isles; that is 60% of the solar radiation found at the equator, and one third of the hot water energy demand of a household. Even on cloudy days we still receive energy from indirect sunlight.

The amount of solar energy reaching the roof of the average UK house would easily provide heating and hot water needs, though the energy available in summer is greater than in winter.

Apart from tidal power and geothermal energy (not to be confused with ground source heat pumps) all renewable energy is derived from solar energy; even the wind and biomass are created as a result of the Sun.

Fig 1. kWh of solar energy received per metre squared per year
(courtesy Solar Trade Association)

What is Solar Thermal Water Heating?
Solar thermal hot water systems (SHW) are one of the simplest and most widely installed of the renewable energy systems in place today. They transfer the abundant energy from the Sun into hot water in your hot water cylinder. SHW will even produce heat in the winter, but will require being topped up to the required temperature by a secondary heating source, such as your existing boiler or immersion heater. In this way, SHW complements your existing hot water system rather than replacing it outright.

For the average domestic dwelling in the UK, daily water heating consumes approximately 5-11 kWh of energy, but this varies considerably with household occupancy and lifestyle.

Space heating from solar thermal is less commonly practised in the UK. This is because a prohibitively large and expensive system would be required. Also when the energy for space heating is most needed (during the winter period) the least amount of solar energy is available, whereas hot water is used throughout the year.
Common types of solar thermal water heating (SHW)
There are two main types of collector available, each with its own characteristics:
•Flat Plate/Flat Panel
•Evacuated Tube with either Direct Flow or Heat Pipes

How does it work?
Solar energy is absorbed by solar collectors, usually mounted on the roof of a building. When exposed to sunlight the solar collectors heat up to temperatures significantly above the ambient air temperature, and once they are several degrees Celsius warmer than the water in your hot water cylinder, heat will be transferred to the cylinder.

  

Fig 2. (courtesy of Solar Trade Association) Evacuated Tube Solar Thermal Collectors


(b) Flat plate collectors on the ECO Centre

The panels themselves usually remain closed (indirect) system filled with an antifreeze mixture (heat transfer fluid) which is circulated around the system. This takes the heat absorbed by the collectors and offloads it into the hot water cylinder via a heat exchanger in the same way as a conventional boiler. No mixing between the water in the cylinder and the antifreeze mixture occurs.

Some installers provide equipment that does not necessarily require the replacement of your current, single coil hot water cylinder, although these are not as common. Direct drain back systems often allow this as the fluid flowing through the collector may be the same water that eventually flows from you hot water tap.
Other systems require a twin coil hot water cylinder (solar cylinder) to be installed, in which the heat exchange coil for the solar water heating system is sited below that of the conventional heating system in the hot water tank. This designates the solar thermal system as the primary water heating source, preventing the conventional boiler from firing unnecessarily.

Flat panels are the simpler of the two collector technologies, and consequently are of lower cost and efficiency. Essentially they consist of a rectangular flat sheet of metal coated with a special absorbent paint. In contact with the back of the metal sheet are a series of pipes through which the antifreeze mixture flows, removing heat from the panel and transporting it to the hot water cylinder. This is encased in an insulated unit and glazed to the front. The flat panels look very similar to Velux windows when installed on the roof.

Evacuated tube collectors are more advanced, containing discreet, parallel rows of evacuated tubes containing a heat pipe, with either a ?dry? or ?wet? connection. Heat trapped within the tubes is prevented from escaping by a partial vacuum and so are more efficient, particularly in colder weather. The heat is transferred from the collector to a manifold, through which the heat transfer fluid flows; taking the heat energy generated to the hot water cylinder.
Once the heat has been delivered to the hot water cylinder, the cooled heat transfer fluid then circulates back to the collector so that it can reheat and begin the process all over again.

What components comprise SHW Systems?
All solar thermal water heating systems contain some basic common components.
1.The solar collector (evacuated tubes or flat panels)
2.Heat transfer medium (antifreeze mixture or water), and circulatory system (pumps and pipes)
3.Hot water cylinder with or without secondary heat exchanger
4.Control units and safety features

How much energy can I expect a SHW system produce?
This depends mainly on the energy availability to the system (collector area, orientation and local conditions) and the amount of water needed to be heated (demand). If little hot water is used, the SHW system will only be able to provide a small amount of energy. If a large quantity of hot water is used, then the SHW has the potential to provide a lot of hot energy into the hot water cylinder, especially in summer. Shading of SHW systems is less critical than with PV.

A properly designed and installed system should be capable of providing between 50% - 70% of the annual hot water demand of the household; up to 100% in the summer months, and down to 5% - 10% in the winter months.

What are the benefits of SHW?
Many tens of thousands of solar domestic water heating systems are estimated to be used currently in the UK. Typically 50-70% of annual household hot water needs can be produced by solar thermal, and provide at least 20 years' useful service. Other benefits include:

•Quick and easy installation, with little disruption to the household
•Free hot water generation
•Compact area requirement ? 3-4 m2 suitable for up to an average four-bedroom house (two flat plates or around 20 evacuated tubes)
•Long component lifetime
•Low maintenance
•Variety of installation methods including roof integrated

Suitability
Ideally, for best performance any SHW system should face between southeast and southwest, and have an angle of tilt of 30-45 degrees to the horizontal for the UK; arrays should not be angled towards the north. Shading should also be kept to a minimum.

Maintenance
Maintenance of SHW systems is low, consisting of an annual check by the householder and a more detailed inspection and replacement of anti freeze by an engineer every 3-5 years.

Installation Costs
A significant proportion of the cost of a solar thermal hot water system is the collectors themselves, with the simpler flat plate design being less expensive. The price of a typical solar thermal system (2-3 panels) installed and commissioned therefore, should be approximately:
•Flat plate system: £2,000 - £4,000
•Evacuated tubes: £3,000 - £4,500