With non renewable energy sources such as fossil fuels predicted to run out within the next 100 years or so at the current rate of consumption, it is vital that other sources of energy are turned to wherever possible. Harnessing the solar energy produced by the sun is one such alternative source of energy. Not only is this a renewable source of energy, it is also clean and environmentally sound, so it's benefits over fossil fuel based energy production are multiple.

One method of harnessing the sun's solar energy is using solar panels to heat water directly. This is usually called solar thermal.

Solar thermal energy systems are designed to absorb solar energy from the sun and to store and distribute it in the form of thermal energy (heat). There are a variety of applications for this technology, some of which are listed below:

• Heating – water and space heating systems.
• Cooling
• Ventilation
• Cooking
• Electricity generation – using steam or gas turbines to generate electricity.

One of the most common uses for solar thermal technology is heating water.

How does it work?

The processes listed below outline how an active solar hot water system operates:

• A solar collector absorbs heat from the sun. This collector can come in different forms depending on the method being used. The two most common collector types are flat plates and evacuated tubes, both of which are installed on a building's roof for optimal collection.
• The heat is transferred from the collector to a working fluid such as water or refrigerant. The transfer is achieved by pumping the fluid through the solar collector when solar radiation is high. An electronic control system is used to determine when this should occur.
• The heat that the fluid has absorbed is transferred to a storage vessel, such as a hot water tank. This is done either directly or by use of a heat exchanger.
• The energy that has been transferred into the hot water tank increases the temperature of the water, thus reducing the amount of power required from a conventional energy system to keep the water at the required temperature. This results in reduced energy bills for the end user.

Benefits of solar hot water

Some of the benefits of a solar hot water system are listed below:

• Cost - the system detailed above can contribute 90% of the required energy for heating during the summer and around 50% during the winter. As this energy is effectively free after initial setup costs, energy bills will be reduced dramatically with a solar thermal water heater installed.
• Reduced carbon footprint – solar thermal hot water systems do not release any pollutants into the atmosphere once installed and are a renewable source of energy.
• Increased hot water tank lifespan – as a result of significantly reduced scaling, a solar hot water system will increase the expected lifespan of a hot water tank.
• Increased property value – having a solar thermal hot water system installed will increase the value of a property significantly due to the desirability of reduced energy bills.
• Government grants/benefits – In many countries governments are running initiatives to encourage the use of renewable energy sources.

Requirements for having a solar hot water system installed

The following are normally required for a solar thermal hot water system installation:

• Around 3 square metres of roof space that is open to the sun for significant periods of the day. This space is needed for solar collector installation.
• Enough room for a larger, or extra, hot water cylinder. This is required to allow a solar heating coil to be installed.
• In some countries planning permission may be required.
• A compatible boiler (most boilers are already compatible with solar hot water systems).

Thermal collector types

Flat plate collectors

These are made up of a flat radiator absorber which is insulated in glass. How efficient they are is dependent on the quality of insulation and the construction method used. Although they can be made cheaply they have disadvantages in that they produce poor performance during cloudy weather and are inefficient at high water temperatures.

Thermal vacuum tubes

These are a newer form of collector. They are made up of evacuated tubes containing black absorber plates. Water is passed through the tubes and insulated by the vacuum thus allowing for higher efficiency and increased operating temperatures when compared to flat plate collectors. They also operate far better during cloudy conditions.

Passive and active systems

The terms passive or active when used to describe a solar thermal water heating system refers to the method that the system uses for water circulation.

Passive

Passive solar water heaters rely on thermodynamics to circulate water through the solar collectors. They are more simplistic than active systems as no pumps are required for the system to operate. This circulation will be affected by sub zero temperatures and therefore passive systems are not recommended for use in areas where such temperatures are common.

Active

Active solar water heaters circulate water or refrigerant around collectors using pumps. Active systems are generally more efficient than passive ones.

Direct and indirect systems

Solar thermal water heating systems are described as either direct or indirect depending on how they actually heat the water within the system.

Direct (open loop)

In direct systems the water that passes through the solar collectors is the same water that building occupants will obtain from their taps or showers. Hot water is pumped directly from the solar collectors to a hot water tank for storage and then distributed around the building in a conventional manner. A disadvantage of this system is its susceptibility to freezing conditions.

Indirect (closed loop)

In this system an antifreeze solution is pumped into the solar collectors rather than water. This solution is then used to heat up the water that occupants will use. A heat transfer unit is used to transfer the heat from the anti freeze solution to the water that will fill the hot water tank. The solution is then pumped back into the solar collector and the cycle repeats. This system is recommended in very cold climates where sub zero temperatures are common.

Common Misconceptions about solar water heating

Below are some of the most common misconceptions about solar hot water systems.

• Solar water heating is not viable in countries with cooler climates such as the UK - this is in fact false, a vacuum tube based solar thermal heating system can provide significant energy bill savings even in areas that lack prolonged periods of sunshine.
• Solar water heating won't provide enough hot water – this is untrue, solar hot water systems can be combined with conventional water heating technologies to ensure that there is always hot water available if needed.
• Solar water heating will not work at night – again this is false, energy is stored during the daytime for usage after dark.
• Solar water heating takes up a lot of space – the space required for a solar hot water system is not significantly higher than a conventional system. The largest part of the system are the collectors which are installed out of the way, on the roof.

Along with thermal, the other most common solar power technology is photovoltaics. Unlike solar thermal, photovoltaics convert sunlight directly into electricity. As a domestic solution solar thermal has a number of advantages over photovoltaics:

• Size – as solar thermal is more efficient than photovoltaics a smaller collector size is needed to generate an equal amount of output. This is obviously of great benefit in any situation where space is limited.
• Efficiency – photovoltaic panels convert only a small portion of the sunlight that reaches them into electricity (around 11%), in contrast solar thermal collectors absorb up to 70% of the sun's heat
• Cost – not only will less solar thermal collectors be needed due to superior efficiency, they are also inherently cheaper than photovoltaic panels. A solar thermal system is much cheaper than a photovoltaic one whilst at the same time being far more efficient.
• Obviously for small scale mobile devices solar thermal isn't practical and photovoltaics reign supreme.