This month the cost and research departments of Mott Green & Wall and Davis Langdon & Everest in conjunction with specialists Solar Century, examine the state-of-play of solar cell engineering applications in buildings.
Solar power is the subject of increasing interest for designers and developers of new build and external envelope refurbishment, especially in towns and cities where other renewable energy options are not generally feasible. Building integrated photovoltaics (bipv) is the term used to describe the integration of solar cells into the building, including its function such as glazing or cladding, together with the arrangement of the other system components and the interface with the normal electricity supplier.

The cost model focuses on construction-related applications and does not address the large and relatively mature market for non grid-connected pv modules, such as remote telecommunications, railway trackside installations and street furniture.

The photovoltaic effect
Photovoltaic materials, commonly known as solar cells, generate direct current electrical energy when exposed to light. Solar cells are constructed from certain semi-conducting materials that absorb solar radiation; electrons are displaced within the material, thus starting a flow of current through an external connected circuit. Conversion efficiency of solar energy to electrical power is improving with advances in technology and ranges from 7% to 18% under standard test conditions. In practice, however, allowing for typical UK weather conditions, an installation of at least 7 m2 of the latest high-efficiency hybrid modules is needed to produce 1000 W peak (1 kWp), which will generate between 750 kWh and 900 kWh per year (enough to provide about 50% of the electrical demand of an energy-efficient three bedroom home). PV devices and systems offer the following potential benefits:

  • As a source of renewable electricity, bipv can contribute towards diversity and security of supply and can help to meet targets relating to sustainability and greenhouse gas emissions. The UK government targets for the energy contribution from all renewables is 3% by 2003, rising to at least 10% by 2010; the current achievement is 2·8%. As an incentive for helping to meet these targets, commercial pv installation owners may be eligible for government grants and/or capital allowances.
  • Power produced from bipv is exempt from the Climate Change Levy. Any surplus power may be exported via the distribution network. Large installations generating around 1 MWh per year qualify for Renewable Obligations Certificates which the building owner may sell in the open market or to the normal electricity supplier, using the procedures introduced recently by the Office for Gas and Electricity Markets (OFGEM).
  • A bipv installation has no moving parts and system maintenance requirements are low; it produces no noise and is therefore very applicable for both densely-populated urban settings and remote rural locations.
  • BIPV systems do not require any additional land.
  • In many cases, pv materials may be used instead of conventional glazing, rain screen or roof cladding, thereby enabling a considerable off-set of capital cost.
  • The systems are highly modular and can be applied to multi-megawatt scale.
  • BIPV is embedded generation, ie it results in no external transmission losses. For many applications, such as offices, the daily solar cycle matches well to the daily operational power demand cycle, so that all of the generated energy may be consumed on site.

The complete bipv installation
The pv modules, roof cladding, rain screen or bespoke laminated glazing account for approximately 60% to 80% of a typical commercial bipv installation. Balance of system is the term used to describe all the other components that make up a complete system. The complete installation is represented diagrammatically in figure 1. Apart from cabling and connections, the main component is the inverter, which takes the dc power from the pvs at its prevailing voltage, converts it to 50 Hz, transforms it to mains voltage and synchronises to the distribution network operator (dno) supply for import/export metering. In practice, the inverter generally consists of a number of modules each with their outputs connected together. The plant room space requirement for inverters is approximately 2 m2 per 100 kWp, similar to other mains power electronics applications. Efficiency is high, at approximately 95%. Balance of system components typically accounts for 20% of the total bipv project cost.

BIPV installation does not involve high-tech skills and, after some basic training, increasing numbers of ordinary roofing contractors are proving competent to handle pv materials successfully. Similarly, electrical contractors cope easily with the system integration, but need to be aware of the 'always live in daylight' and working at height risks. Useful guidance documents on design, installation, safety and cdm aspects have been published and may be downloaded from www.dti.gov.uk/renewable/ index.html

For systems larger than 5 kVA, The Electricity Association Engineering Recommendation G59/1 applies, and the distribution network operator may require the witnessing of certain commissioning tests; for systems smaller than 5 kVA, Recommendation G77 applies.

PV technology and the markets
Monocrystalline and polycrystalline silicon modules have been used since the early 1970s and they have proved to be reliable, with low maintenance needs. Traditionally, silicon has been recycled from the electronics industry. This still accounts for more than 50% of silicon solar cell production, but raw silicon is increasingly being used as feedstock as the market grows. Considerable cost reduction is likely to result from the scaling up of methods used to grow silicon crystals in sheet form, and from the mass production technologies that are already producing thin-film cell structures using amorphous silicon (a-Si) deposited on glass or stainless steel substrates. Table 1 shows the main characteristics of several types of currently available silicon-based technologies.

Renewable generation issues
One issue is the value of bipv generated energy. Every kWh of energy generated by the pv installation and used in the building represents a cost saving, typically between 4p and 6p at today's prices. Most bipv is grid-connected with import/export metering so that any surplus energy can be purchased by the electricity supplier, thereby producing income for the owner. Energy costs are predicted to rise in the UK as fossil fuels dwindle and nuclear power stations expire, so that the value of the contribution from bipv is likely to grow.

A second issue is that of Renewable Obligation Certificates (ROCs). These were recently introduced by the Office for Gas and Electricity Markets (OFGEM) as a means for electricity companies to prove that they are supplying a certain percentage of power from renewable sources; the target being 10% by 2010. Owners of bipv installations generating more than 1 MWh per year may gain OFGEM accreditation as renewable generators and will be able to sell on their ROCs to the electricity supplier as part of the supply contract. The value of ROCs depends on how well or badly electricity suppliers are doing at meeting their renewables targets on a monthly basis. A reasonable estimate for the rest of 2002 might be £50/MWh.

A third issue is energy conservation through awareness of bipv. Studies have shown that in buildings on which pv is installed, and in which the quantity of energy generated is prominently displayed, staff and other users are more conservative in their use of energy than those in a similar building without bipv. Savings of between 5% and 10% are consistently reported, and it seems reasonable to reflect this saving in any investment appraisal.

BIPV project costs and savings
The project considered is an indoor training centre with a large roof area, a very well insulated envelope and low energy requirements. There was a 65% grant as it is a DTI large scale demonstration project and economies of scale are achieved as it is the largest pv roof (by area) in the UK. Table 2 gives a breakdown of costs.

  • Location: Alexander Stadium, Birmingham.
  • Project status: in construction, for completion December 2002.
  • PV technology: Kaneka LSU 202 thin film amorphous silicon modules.
  • PV construction: custom frames to integrate aesthetically with KalZip roofing.
  • Conventional material displaced: none, although the pv will provide shading and thereby a reduction in building heat gain.
  • PV installation area: 1500 m2.
  • BIPV system rating: 102 kWp.
  • BIPV useful energy generated: 80 MWh/year.
  • % of total annual site electrical energy generated by bipv: in excess of 100%.

Conclusions
BIPV installations have the potential to make a considerable contribution towards the renewables targets by 2010 and associated reductions in greenhouse gas emissions.

Initial costs are high, but may be partly offset if the pv is integrated with the building fabric, thereby displacing conventional materials such as rain screen or roof cladding and curtain wall glazing.

The financial return from bipv is low, but is very sensitive to the price of electrical energy and ROCs, both of which are widely predicted to rise. However, the 'added-value factors' derived from the promotion of sustainability are increasingly recognised and have resulted in marketing advantages across several commercial sectors.

The scale of the bipv industry in the USA, Japan, Germany and several other countries suggests that the UK is lagging the leaders by perhaps 10 years or more. It is anticipated that the renewables obligations, together with government grant incentives, may lead to the development of leapfrog technology breakthroughs, enabling a significant expansion of the UK bipv industry, including manufacturing.

Acknowledgements; Mott Green & Wall would like to thank Solar Century (www.solarcentury.co.uk) for their assistance in the preparation of this cost model.