Concern for the environment, combined with rising fuel costs, means that more and more people are looking to alternative sources of energy to power and heat their homes ‘microgeneration’, in the current buzz term. While solar thermal, photovoltaics, heat pumps and other forms of green energy are still far from mainstream in some parts of the world, they are no longer seen as solutions for only a handful of dedicated eco-pioneers.
Solar thermal is a tried and tested technology that uses the sun’s energy to heat water. It’s a rapidly expanding area of renewables, growing at an estimated rate of 50 per cent per year. For some years rooftop solar panels have been a common sight in sunny Mediterranean countries. In 2006 Spain passed a law that states that all new housing must have solar thermal collectors. But even in more northerly regions solar thermal can be very effective. Marstal, in Denmark, gains all of its hot water in summer and a third of its overall heating requirements per year from a solar thermal plant on the outskirts of the town. In the UK a solar thermal system is capable of meeting 60-70 per cent of domestic hot water needs per year. Although solar thermal systems are most economic when they are installed in new houses, especially as integral building elements, the are also highly suitable for retro-fitting.
The main element of a solar thermal system is a collect that captures the sun’s energy and heats liquid piped through it. There are three types of collector: the flat-plate collector, which is the most familiar, the excavated-tube collector, which is more expensive and more efficient but can be slightly problematic, and the unglazed-plastic collector, which is principally used to heat swimming pools
There are two types of heat distribution. Direct circuits heat water from household taps. These are more common in warm climates where there is little risk of temperatures dropping below freezing. Indirect circuits, which are more typical in the UK and other temperate regions, use a separate circuit to transfer heat from the collectors to the water cylinder. Most of these are pump- assisted and controls ensure that the system only operates when the solar panels have generated enough energy to heat the water in the cylinder.
Solar thermal systems either require a separate pre-heat cylinder positioned between the cold water feed and the existing cylinder, or the replacement of the existing cylinder by a large cylinder with two heat exchangers or coils, onc connected to the solar panel and one to the conventional boiler. Size of the pipework is also critical. If pipes are too wide, heat can be lost in transfer .
The siting and angling of solar collectors is critical for efficiency. Panels can be fitted to roofs, on adjacent buildings or in the garden. They must face within 90 degrees of south (in the northern hemisphere), which basically means any position from southeast through to southwest. Tilt angles vary according to latitude. In the UK, the optimum tilt angle is 35 degrees to the horizontal. It is also essential that the collectors are not overshadowed by trees or existing buildings.
Another type of solar generator is the photovoltaic cell or PV, which, as the name suggests, converts the sun’s energy into electricity. PV cells, made from silicon, are grouped to form modules, which are then arranged in panels or arrays of panels. Solar tiles and slates are also available in a range of colours, along with transparent cells or film that can be installed on glazed roofs or within windows.
Huge advances have been taking place in this area of technology over recent years. One new development is a complete roofing system that integrates solar thermal and PV. The tiles are installed in the same fashion as ordinary tiles. Another potential breakthrough, which could see windows functioning as powerful solar panels in the near future, has emerged from research carried out at the
Massachusetts Institute of Technology. Transparent dyes in the glass capture and redirect light to PV cells in the window frame where it is converted into electricity. Because far fewer cells are required, the price of the technology could plummet.
Sunlight falling on PV panels is converted into electricity. Because there are no moving parts, the generation is entirely silent. The electricity produced is direct current (DC), which then has to be converted to alternating current (AC) in an inverter. The PV system must be connected to the
mains supply via a fuse box for safety.
During the day, the PV system generates electricity continuously, with the surplus or spare capacity flowing into the grid. At night, when electricity is not being generated, or at times of exceptional demand, power can be imported back from the grid. Even in countries where households receive only a relatively small amount of money for the surplus electricity they produce, it is possible to make net savings. Where the ‘feed-in tariff is higher, the financial incentives are even greater.
PV systems can also be run off the grid, in which case surplus electricity is stored in a battery that acts as the main power supply. An inverter is stll required to convert the DC electricity to AC.
Like solar thermal panels, PV panels can be installed on the roof, on the ground and also on south-facing walls. Maximum output (in the northern hemisphere) is achieved with a south-facing orientation. Optimum tilt angles vary according to latitude. In southern England, for example,
a 30-degree tilt is best, whereas in Scotland the angle increases to 40 degrees. Obstacles such as high surrounding buildings, chimneys and trees that shade the panels will reduce efficiency. PV tiles or slates that are integral to the roof structure are much less intrusive.
Systems must be sizecd, designed, detailed and installed electrical wiring, placement of junction boxes and detailing by accredited professionals. Issues include: where to run of supporting frameworks.