Solar Water Heating

Green Energy > Solar Water Heating

The sun’s energy can be harnessed effectively for high energy water heating.

Of all the renewable energy technologies, solar water heating has arguably proven itself to be one of the most reliable and cost-effective. It’s already used by millions of households across Europe, and while the UK and Ireland have been slower on the uptake, government grants are helping to raise the number of adopters.

Solar water heating systems are standalone systems that collect solar radiation, which is converted to heat. This heat is then used to pre-heat water in a water cylinder, either directly or indirectly by means of a circulating fluid. They can use indirect sunlight and so will work even in the north of the British Isles and Ireland. On the downside, they are less effective in winter months when hot water needs are likely to be greater. A backup heat source is needed to make up for this shortfall. Unlike photovoltaic cells, they don’t produce electricity and cannot be connected to the grid.

Indirect systems have three components: the solar panels which collect the solar radiation, a heat transfer system, and a hot water cylinder. Once the panels collect the radiation, the fluid in the heat transfer system transfers the heat to the hot water cylinder. Even in winter, the system can heat up the water to around 30ºC. In summer, temperatures of over 80ºC are possible. Solar water heating can provide around 50-65% of the total energy required although an auxiliary system such as a boiler or electric immersion heater is still required to heat the water to a useable temperature.

There are two types of solar energy collectors, each with slightly varying amounts of capturing potential. A flat plate collector contains a dark plate that sits inside an insulated box and is protected by a glass or durable plastic cover. The plate has a selective coating which ensures higher absorption and lower emission of sunlight. These type of collectors typically capture energy within a range of around 380-450kWh.

Alternatively, evacuated tube collectors use glass vacuum tubes to encase a series of metal strip collectors. These tubes are more efficient and have low heat losses, so tend to work better in cold weather. On average they can capture about 500-550kWh of power and their superior efficiency is reflected in their slightly higher price.

The amount of roof space needed to install solar water heater collectors is typically only around three to four m2 for an average household. This is slightly less than the space required for photovoltaic cells. The solar collectors can be mounted so that they’re flush with the roof and look just like a skylight. Positioning guidelines require a south facing roof where possible, but southeast or southwest will also work. A 30-45º pitch is ideal, although the panels can normally be orientated to face the sun if the roof is flat. There should also be minimal obstructions from other buildings or trees.

Solar water heating may not be as versatile as photovoltaic cells, but they offer a better cost to savings ratio on the initial investment. They can also be particularly valuable in areas that are off the gas grid, due to the large differential between gas and electric energy costs.

Like photovoltaic cells, solar water heating harnesses the free and clean energy of the sun, which reduces the use of fossil fuels and damaging carbon emissions. A domestic system will provide around 1,600-2,000kWh of clean energy a year, which equates to a reduction of CO2 released into the atmosphere by around 400-1,000kg.

Finally, solar water heating is a well-developed technology which means there’s a vast range of equipment and a good number of installers to choose from. Maintenance is also minimal, with nothing much more than a cursory system check needed every three to five years.

Solar water heating systems can, cost effectively, meet a large proportion of a household’s energy needs. They use a good, clean energy source, have proven to be reliable and can reduce energy bills significantly. This is a renewable energy technology worth looking at more closely.

For more guidance on adopting this system, click here.