Building in solar technologies

Solar technologies can generate either electricity – from photovoltaic cell panels, or hot water – from solar thermal panels.

Richard Hanson

Solar thermal

Solar thermal technology is generally used for the production of hot water and consists of roof-mounted panels that can efficiently capture heat energy from the sun. In the UK, solar thermal panel systems can provide around 50-60 per cent of the annual energy requirement for domestic hot water in dwellings. Summertime operation of the system is normally sufficient to meet full hot water demand, but in the winter supplementary heat from other sources is required. A larger panel area can be used to contribute to space heating as well.

A low temperature underfloor space heating system makes an excellent companion to solar thermal technology as they use larger volumes of distribution fluid at lower temperatures than do conventional central heating systems with radiators.

The panels can also be used as part of a community heat network at a neighbourhood scale. It is estimated that cost savings of up to 20 per cent may be achieved through bulk purchase and installation. The available roof area of buildings served may limit the area of solar thermal panels that can be integrated into a particular scheme. Solar hot water heating is also appropriate for mixed use schemes or commercial applications where there is a significant hot water demand.

Larger panel areas can provide excess summer-produced heat for possible storage in interseasonal thermal storage systems for use in the winter. Drakes Landing in Canada is the most famous example of this approach.

The main types of solar water heating system used in the UK are flat plate collectors and evacuated glass heat tubes. Manufacturers’ figures show that in London energy produced from solar water heating is an average of 401kWh/m2/yr for flat plate collectors and 518kWh/m2/yr from an evacuated tube system. For either type of collector, the most benefit can be gained if they are mounted facing within 45 degrees of south, at an angle of around 30 degrees. Other mounting positions are feasible, but output will be reduced.

Solar thermal panels are by far the most popular renewable installation in Britain, partly because users feel empowered by them and they are easy to understand.

They can be well designed and are relatively easy to integrate into the architecture of the roof and could therefore be considered a basic requirement in new homes. Importantly, however, unless feeding into a shared Interseasonal Thermal Store, they are not generally compatible with other technologies that require significant summer heat sales to homes in order to be viable such as CHP. They are most appropriate at the scale of individual systems for dwellings or communal systems for blocks or buildings.

They can be an important retrofitting measure for existing residential buildings as they can be relatively easily installed into existing roofs.

Photovoltaics

Photovoltaic panels (PVs) are also most appropriate at the building scale - either individual systems for homes or communal systems for blocks. Although currently expensive the emerging development of ‘thin film technology’ has the potential to change this. As with solar thermal technology savings of up to 20 per cent may be achieved through bulk purchase and installation.

The technology works through solar cells or solar photovoltaic arrays that convert the sun's energy into electricity through photon-to-electron energy transfer, which takes place within the dielectric materials that make up the cells. Photovoltaics typically used in the UK can generate some electricity even on overcast days although peak output is attained around midday on a sunny summer’s day.

They offer a simple, proven, elegant, relatively maintenance free solution to generating renewable electricity, especially if incorporated as a building integrated photovoltaic array (BIPV), for example as part of a roof or façade.

Photovoltaic panels may be connected to export electricity to the grid if there is a surplus of electricity, although this may only prove economical for large arrays due to grid connection charges.

In the UK, the maximum total annual solar radiation is usually found at an orientation of due south and a tilt of 30 degrees. The typical annual output from an unshaded one square metre array in such a position is in the range of 50-150kWh per year. Mounting panels on unshaded, south facing roofs (or vertical façade areas) will maximise the output and give all year round best performance. Panels will still be effective on east west roofs but will function with reduced output of approximately 20 per cent.

Photo by Simon Doling/Feilden Clegg Bradley

The Heelis Building

The National Trust’s Heelis Building has used solar technology to steer the design of the overall building.

The building’s form and orientation maximises the north/south orientation of the roof pitches. The north facing pitches allow the maximum possible amount of daylight to enter the building without causing it to overheat, while the south facing pitches are covered in solar panels which provide around 30 per cent of the buildings electricity needs.

Read more about the Heelis Building.

Most photovoltaic products have a lifetime of around 20-30 years. Ongoing maintenance costs are reasonably low since there are no moving parts. However, regular inspections of the inverter are necessary and panels require cleaning to eliminate deposited dust that reduces the efficiency of the system.

Scale

High density development means there is not much roof space per dwelling or person which limits the application of solar technologies on buildings in dense urban areas. Using the current zero carbon compliance definition for homes there is only usually about a fifth of the roof area you need for the solar panels and therefore solar technology can often only provide a portion of the heat or power needs in high density developments.

An additional constraint is the need for self cleansing roof glazing which means panels cannot be placed any flatter than 32 degrees as this limits the velocity of rain run off and hence its ability to play its role in the self-cleaning process.

Cost

The average capital costs for a solar water heating system would be £850 per square metre. The capital cost for a photovoltaic system would be between £4,200 and £4,800 per KW(peak)e. These costs reflect the capital cost when applied to a Part L1a 2006 compliant home and is based on figures from 2008 Communities and Local Government research. The same research also undertook an economic cost and benefit analysis of each technology. This found that the value of saving in energy costs for solar water heating systems was £2,324 per tonne of CO2 saved.

However, once the displaced carbon calculation that is part of the Part L regulations is removed costs for photovoltaic systems are likely to be about a third higher than those quoted here.

There are important cost benefits in matching photovoltaic energy production to demand. The role of thermal masterplanning in considering this as an appropriate technology is therefore important.

Photo by Gorgues Xixons SL

Solar panels in Barcelona

The City of Barcelona introduced the compulsory installation of solar collectors and the use of low temperature solar energy for the production of hot water for household and office use in new and remodeled buildings. The city has stipulated design and quality standards for installation and has established a range of fines for non-compliance.

Read more about solar panels in Barcelona.

Priority: develop a low carbon and renewable energy portfolio
Tags: energy, neighbourhoods, buildings and spaces