Using the energy of the sun is one of the most effective strategies of the renewable technologies, in part because buildings can benefit from both passive and active systems.
Passive solar design can provide between 20% and 40% of the energy needs of a typical house by providing useful space heating. Passive solar design is a key part of the Passivhaus method by reducing heating loads of the building.
The suns energy can also be used to generate both electric and heat energy for our buildings. Known as active solar strategies they include photovoltaic panels (PV), photovoltaic thermal (PV-T) and solar thermal panels.
Photovoltaic panels generate electricity by converting sunlight directly into direct current electricity. The cost of PV technology is decreasing, and performance improving, resulting in this renewable technology becoming increasingly popular.
Traditionally PV arrays were installed on existing buildings, usually fixed onto an existing roof. Now, with the advancement of technologies to include integrated roof tiles, glass curtain walling and other in roof glass panels, we see PV being specified on a wider more varied selection of new build projects.
Just 10-15% of the suns energy is converted into electricity when using a photovoltaic panel. Around 30-35m2of PV would be required to meet a typical dwellings entire electrical need.
The efficiency of the photovoltaic will depend on many factors which all must be considered during the careful design process.
- Panel orientation is key to successfully achieving optimum performance (see table). Within 10˚ degrees of south (up to 30˚ is acceptable) and panel angle between 30˚ and 40˚ degrees delivers the best output.
- Any shading must be avoided which includes trees, other buildings, chimneys, parapets – any of these can significantly decrease performance.
- Access for maintenance and cleaning must be considered, on flat roof design a 1m walkway around the panels must be kept clear.
- Structural weight and wind loads must be considered as part of the design.
- PV require good circulation around the panel to avoid overheating which will have a significant effect on generation. At least 150mm air circulation is required.
- Light roof surface to avoid overheating.
Following table shows the effect of tilt and orientation on energy output.
- Any generated electricity that isn’t consumed can be fed to the national grid (feed in tariff)
- On low generation days the energy can be bought back at low rate
- Few moving parts
- Can be integrated into the building fabric
- Panels set at different angles can achieve optimum efficiency at different times of the day or year
- The fuel (sun) is free and there is no noise or pollution created from the PV panels.
- The systems have long service lifetimes.
- Intermittent supply of sun
- Difficult to store the generated electricity
- Expensive to purchase and install
- Long payback time
- Requires careful design to maximise performance
- Requires specialist fitting
- Must be installed away from shade or potential future shade
- Require regular cleaning to ensure maximum performance
- Generate full capacity 10-30% of the time
Photovoltaic Thermal (PV-T)
PV-T are panels which general both electricity and heating. They are a fairly new technology and currently are relatively untested. They have the potential to produce more energy than a standard PV by using the overheating of the panels to its advantage, drawing heat to be used as a heat source for the dwelling. This would be achieved using an air source heat pump. This system can produce around 2 times more heat than electricity.
Solar thermal is one of the most popular low and zero carbon technologies. It is seen in both new builds and retrofit dwellings alike. The solar thermal system collects energy from the sun which is used to heat up hot water.
There are two main types of solar thermal systems, the flat plate collector and the vacuum tube collector.
The flat plate collector consists of a network of black painted pipes in an insulated bed, within a box that is glazed on top. They are best suited to situations where the demand is for lower temperatures between 30˚ and 70˚ degrees.
The vacuum tube collector is up to 40% more efficient than the flat plate collector and is able to generate higher temperatures. The collector heats up liquid contained in the tubes which is then transferred to the hot water system.
The solar thermal system will usually need a supplementary heating source as it is unable to provide all of the hot water needs throughout the year. The systems are normally sized to meet between 50-70% of a households hot water needs.
The most common solar thermal system to be used in the UK is the vacuum tube collector, which is what we will focus on here.
- Solar water heaters need to be positioned on a south facing incline. It is said that a solar thermal system can be effective up to 45˚ degrees east or west.
- The collector needs to be at a tilt of around 35˚ degrees, for optimum performance.
- Shading must be avoided to reduce any losses in performance of the system.
- Hot water tank is usually positioned in the roof space which needs to be accounted for both in terms of access and structure.
- The storage cylinder need to allow for about 40-60 litres per person, with a minimum of 80 to 100 litres of storage per m2of collector. A typical size for a family of four would be a cylinder of 200-300 litres.
- The tank requires insulation of at least 100mm.
- Dark roof surface to increase heat harnessing.
- The system works well to be combined with underfloor heating or similar
- During the winter around 20% of hot water needs will be met, while in the summer, all hot water needs should be met.
The UK is behind many other countries in terms of solar energy and use of PV, partly due to lack of government incentives but also the price of alternative energy sources. Hopefully this will begin to change as technologies are developed, they become more affordable and more efficient.
Solar Energy References:
Environmental Design Pocketbook – Sofie Pelsmakers
Construction Technology – Chudley – Greeno
Rough Guide to Sustainability – Brian Edwards
Green Building Guidebook for Sustainable Architecture – Bauer Mosle Shwarz