Sharing our experience

A place in the sun Lessons learned from low carbon buildings with photovoltaic electricity generation

Contents

Sharing our experience 01

Photovoltaics 02What are photovoltaic modules?

Why choose photovoltaics? 04The benefits of photovoltaics

Assessing site feasibility 07How to assess the suitability of your site, including solar access, planning and technology choices

Procurement and installation 11 Why gathering the right experience, setting up contracts, team dynamics and cost control matter

Ensuring best performance 14Factoring metering and maintenance into the earliest design stages

Most of the case study projects that installed photovoltaics are saving between 5% and 10% carbon per year. They would stand to save £3,700 per year through the feed-in tariff

1A place in the sun

Sharing our experience: About this booklet

‘A place in the sun’ is part of the ‘Sharing our experience’ series. These booklets provide advice and tips to help you to plan, build and manage cost-effective low carbon buildings that really work to save you money and carbon.

The insights are based on real data from 28 case studies from the Department of Energy and Climate Change’s Low Carbon Buildings Programme and our work on refurbishments. The projects cover many sectors including retail, education, offices and mixed use residential buildings.

Further information

To find out how we can help with your low carbon building project, contact us on 0800 085 2005 or visit www.carbontrust.co.uk/buildings

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2A place in the sun

PhotovoltaicsPhotovoltaic (PV) modules are panels of solar cells which convert sunlight into DC electricity. This is then converted into the AC electricity used in buildings.

Site and orientation

PVs are reliable, low maintenance and silent. They are an efficient source of zero carbon electricity for new build and refurbishment projects. Ideally, PV arrays should be free from shade, face within 45° of south and be inclined at an angle of 30° of the horizontal plane.

Partiallyshaded PV

Unshaded PV

35-40°

SE – SW

Shade

W

S

E

70 65 5065

100 95 9095

% of maximum output

Figure 1 Shading, inclination and orientation impact PV array efficiencyMENU

3A place in the sun

Mounting

A number of modules connected together are referred to as an array. PV modules can be designed into the building envelope, for example, replacing walling, cladding or roofing components, or can be mounted separately on purpose-built frames. Thin film solar cell can be applied to materials such as glass or metal.

Generation

PVs work well for buildings where electricity is needed year-round in daylight hours, as electricity generated can contribute to a building’s daytime demands. If more electricity is generated than the building needs, this energy can be exported to the National Grid.

Frame-mounted PVs on the roof of Stoke Local Services Centre (left), and building integrated PV into rooflight glazing at the London School of Hygiene and Tropical Medicine

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4A place in the sun

Why choose PVs?If you have enough space for installation, access to sunlight, and no risk of overshadowing from nearby buildings or trees, PVs may be a viable choice for your project.

Motivations

When we asked our project teams why they chose PVs, we received a variety of responses:

• Once installed, PVs are silent and easy to run.

• Highly visible panels can make a bold sustainability statement.

• Equally, panels can be integrated or hidden where appearances matter.

• They are a tried and tested technology with low maintenance costs.

Funding and payback period

None of the case study projects qualified for the feed-in tariff as they received grant funding. If The City Academy, Hackney, for example, had

been eligible to receive the feed-in tariff, it would have received £3,690 a year, based on a generation tariff of 31.4p/kWh and an export tariff of 3p/kWh. This income is in addition to an estimated £1,050 in electricity savings.

The payback period for The City Academy is approximately 29 years. The payback for Stoke Local Services Centre is 16.5 years. The payback periods for the case study projects would be shorter if current capital costs are used and if the panels were all achieving their theoretical performance (i.e. were not overshadowed and were facing directly south):

• The capital costs for four of the case study projects were between £6,200 and £6,500/kWp (a measure of system size). Current capital costs are less than £4,500/kWp.

• The performance of the panels at Hackney is 552kWh/kWp when it could, theoretically, achieve 850kWh/kWp. Stoke is achieving a good performance.

Taking into account these two factors would reduce the payback periods to 23 years for The City Academy and 12 years for Stoke.

Feed-in tariff

The feed-in tariff (or clean energy cashback) provides an additional incentive to install PV. If you opt in you will be paid for generating electricity from PV and receive further income for exporting electricity to the National Grid.

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5A place in the sun

London School of Hygiene & Tropical Medicine’s South Courtyard Development view of the atrium looking up to the BIPV roof

Costs

Installed costs ranged between £6,200 and £6,500kWp for standard ‘bolt-on’ types of panel.

The costs increased quickly on projects where:

• access was difficult

• PV was integrated into other building elements

• PV was used for aesthetic enhancement.

Live energy information

The City Academy engaged and educated pupils, teachers and visitors by installing a display panel in the school demonstrating the cumulative CO2 savings from the PVs and other low and zero carbon (LZC) energy sources on the site, with ‘live’ information about energy consumption.

Carbon reduction

The projects we looked at found their PV installations contributed to reductions in CO2 of up to 10% compared to 2006 building regulations.

Several sites were motivated by the fact that PVs are easy to monitor and can provide ‘real time’ data on their effectiveness.

Provided PVs were installed correctly and overshadowing was avoided, they were found to perform as expected or better.

The installations at One Brighton and Dandridges Mill were tested and proven to generate levels of electricity as good as, if not better, than those predicted at design stage.

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6A place in the sun

Made to fit

In one project planning constraints meant PV panels had to be integrated within the rooflight system. This was more expensive than predicted as the design was bespoke, so a number of non-standard panels had to be manufactured. This increased the capital cost to £33,500/kWp.

The City Academy’s original renewable energy strategy included a solar thermal system, but this had to be rethought during the tender stage due to escalating costs. They therefore considered PV and found it was significantly more cost-effective in terms of money spent per kgCO2 saved than solar thermal and ground source heat pumps (see Figure 2).

The total installed capital costs of PV per kg of CO2 saved can be lower than other renewable energy technologies.

Lessons learned

• The PVs performed well when correctly installed and when they weren’t overshadowed.

• They are silent and easy to run and maintain.

• Surplus generation can be sold back to the Grid and financial incentives are in place.

• When comparing CO2 reductions against money invested, PVs can compare favourably to solar hot water and ground source heat pumps.

• PVs can be used to make a statement, can be integrated into the building, or hidden from view depending on the demands of the project.

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7A place in the sun

Assessing site feasibilityThe success of PVs depends on the suitability of the site. It is important to consider overshadowing, mounting and the visual impact of the installation.

Predicting output

It is easy to quickly estimate output from a solar array, once you’ve assessed the potential location, using the manufacturer’s data on system efficiency and the solar data as shown on Figure 2.

Figure 3 shows a comparison of the predicted and the monitored energy output from the PV panels for the case study projects. Three case studies overestimated the energy output and two underestimated the use. The overestimates are caused by a combination of the assumptions and modelling tools used to predict energy output, and some underperformance due to overshading.

Figure 2 Map showing available solar energy across the UK

750

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850

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950

1000

1050

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kWh/m2/year

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Avoiding overshading

Consultants and specialists are able to make fairly robust predictions about the performance of PV for most building types and locations.

However, it’s essential that you consider the effects of overshadowing from trees and neighbouring buildings. Even a small amount can dramatically reduce the electrical output of a PV array.

At Fairglen some housing units could not be fitted with PV panels because they were in the shade of existing trees.

At Stoke Local Service Centre the final panel positions were determined by solar shading analysis, taking into account surrounding buildings and the Clock Tower. 0%

1,500

3,000

4,500

6,000

7,500

9,000

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13,500

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En

ergy

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Predicted energy output (kWh/yr) Monitored energy output (kWh/yr)

Hackney Academy, London

Fair Glen, Phase 1 (two houses)

One BrightonLondon School of Hygiene and Tropical Medicine

Stoke Local Services Centre,Stoke-on-Trent

5,400

10,122

1,915 2,101

15,000

11,641

5,750

2,500

7,930

6,936

Figure 3 A comparison of the predicted and the monitored energy output from the PV panels for the case study projects

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PV panels at Stoke Local Service Centre are on frames to achieve optimum inclination and solar access. The panels are hidden from view.

Mounting

PVs are a very flexible option. They can be incorporated into buildings using:

• roof-based systems

• façade systems

• shading devices.

When PVs are added to an existing building where the orientation and solar access are already fixed, they can sometimes be mounted on special frames to make sure they receive maximum sunlight. The cost of framing and integrating the panels onto the roof for Stoke Local Service Centre was approximately 26% of the total capital cost (£52,000).

Dandridges Mill, The City Academy and Stoke Local Service Centre used purpose-built roof frames to ensure their PVs were south facing and 30° to 35° from the horizontal plane.

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10A place in the sun

Roof-mounted PV panels at Dandridge’s Mill (a Grade II listed building)

Visual impact

If you’re considering PVs you’ll need to decide whether you need to minimise the visual impact to satisfy planning constraints – for example, in conservation areas or on listed buildings – or to enhance it to promote the sustainable nature of the building.

For Stoke Local Service Centre, planning permission was straightforward because the local authority endorsed the project’s low carbon aspirations.

Dandridge’s Mill, a Grade II listed building, won approval for panels after they were shown to be an integral part of the low carbon strategy.

The South Courtyard Development at LSHTM is surrounded on all four sides by Grade II listed buildings, leaving the roof as the only viable place for an array. For aesthetic reasons, the planning authorities insisted on a Building Integrated Photovoltaics (BIV) solution, but the client raised concerns that this would block the view from the building and prevent daylight from coming in. Although it diminished the electrical output, the designers reduced the density of the PV cells within the laminated roof glazing, retaining the daylight and views and overall aesthetic appeal of the designed space.

Lessons learned

• It is quick and easy to predict the output of a PV installation.

• It’s important to assess solar access through solar modelling and/or site investigations.

• A small amount of overshading can dramatically reduce performance if you don’t compensate for it in the design of the array.

• PVs are flexible, with multiple installation options.

• Frame mounting can allow elevation or rotation for optimum exposure.

• Visible PVs are becoming increasingly acceptable – even desirable.

• Speaking to planners early allows you to resolve any aesthetic conflicts.

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11A place in the sun

Procurement and installationFinding experienced delivery partners, fostering good relationships and setting up collaborative work practices are all essential to project success.

Building successful teams

To achieve a successful project it’s essential that your engineers and contractors have experience in designing and installing PVs.

At The City Academy the design team remained mostly the same throughout the project, and the expertise of the consulting engineers added considerably to the success.

The way that contracts are managed can also make a difference.

Contracts

At Stoke Local Service Centre the fact that the contractor was appointed via the framework agreement meant that he was able to provide valuable input early in the design to manage risk and improve the efficiency of the programme.

All work packages, including renewable energy systems, were competitively tendered through the main contractor.

The City Academy used a Project Partnering Agreement (PPC2000) to foster mutual trust and co-operation through the design, supply and construction processes.

Coordination

Briefing contractors about the overall design of the building enables them to take other aspects of the design into consideration. One project suffered from a lack of co-ordination during the early design stages and it later transpired that roof-mounted ductwork overshadowed the PV array. This reduced the amount of electricity produced.

The PV array is overshadowed by ductwork and access stairs

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Controlling costs

Early involvement of an experienced contractor, quantity surveyor and engineer will help to give better predictions of capital costs.

The tender process needs to balance experience and knowledge with cost.

At One New Brighton, planning constraints meant arrays could only be mounted on one building. Fortunately, the design team was able to source higher performing panels and generate sufficient electricity from one rooftop array. The higher performing panels cost more, but a lower specification would have compromised the entire scheme.

One New Brighton PV array with simple frame

Modules and equipment

Controls and equipment

Installation

Builder’s work

Commissioning

11%

8%

22%

50%

9%

Figure 4 Cost breakdown for PV installation at Stoke Local Service Centre

Engage contractors early and allow them to inform the design

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Disparity between tender costs and final costs is often down to overlooked ancillary items from specialist equipment, installation and commissioning costs to meeting planning requirements and developing bespoke design solutions.

At Stoke Local Service Centre the additional cost of controls and equipment, installation, builder’s work and commissioning were roughly equal to the cost of the PV modules themselves.

On another project, the bespoke nature of the integrated system demanded by planning meant that final costs were as much as five times those predicted at earlier stages.

Lessons learned

• Organise tendering processes to score on experience and commitment as well as cost.

• Choose contractors and suppliers with experience of similar scale installations.

• Ensure all suppliers and contractors are briefed on the design intent and bigger vision.

• Bespoke solutions and ancillary items can cause costs to spiral.

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14A place in the sun

Ensuring best performancePeople often overlook the need for commissioning, monitoring and engaging the Facilities Management team. All three are vital to ensuring the building works as intended.

Metering

The projects demonstrate that successful commissioning, monitoring and running of any PV system is assisted by a good submetering strategy – that is, a meter to monitor the electrical output from every array.

Fairglen used submetering to assess the performance of the renewable technologies in a domestic setting so they could identify technology choices for future sustainable housing developments.

At One Brighton, the Green Caretaker has to manually read the PV meters each month and divide the savings between dwellings. This could have been managed by an automatic submeter.

Figure 5 A typical daily report showing building integrated PV output over a peak day MENU

15A place in the sun

Commissioning

Commissioning the PVs at the case study projects was generally very quick and easy and took less than a day. Generally, ongoing commissioning is not necessary for PVs.

While commissioning PV systems is relatively simple, it was found to be worthwhile to keep specialists involved through the commissioning stage.

Proper commissioning helps ensure the PV’s electrical systems are safe. On one project, the commissioning engineer suggested installing railings on the roof to ensure inspections could be safely carried out.

At The City Academy, commissioning was shared by the electricity distributer and the specialist PV installer to make sure both parties were happy with safety measures.

Handover and monitoring

A PV system and the metering and monitoring system may require fine-tuning to ensure readings are calibrated properly.

Monitoring allows you to check that the PVs are generating to predicted levels, and helps identify shortfalls or faults.

Monitoring was an important consideration at Stoke Local Service Centre. Local inverters display the energy generated from each panel and overall consumption data is converted and logged onto the BMS.

Early collaboration between the designers, PV suppliers and commissioning specialists will ensure an integrated approach towards successful installation and operation of the PV system

Maintenance

One of the appealing aspects of a PV system is that maintenance requirements are low. However, the facilities management still need to clean the panels periodically, monitor performance and check the electrical installations.

The performance needs to be monitored to ensure that the panels and inverters are still functioning. Inverters have a typical warranty period of between five and 10 years, with some covered for 15 years, but there were some early failures documented in case study projects.

As a rough guide, inverters cost between 5% and 10% of the system cost. Some of the case study projects have set up a fund to cover replacement costs as part of the maintenance budget.

The panels’ performance degrades by 0.5% per annum. The PV modules are typically covered by a 25-year limited warranty of 80% power output or a 12-year limited warranty of 90% power output.

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Constant supply

The electricity supply from PV modules cannot be switched off, so you’ll need to take special precautions to make sure that live parts are not accessible during routine maintenance.

At The City Academy the building’s facilities manager has allowed £500 per annum to carry out the following maintenance:

• modules cleaned every six months

• output monitored

• periodic electrical inspections.

This accords with the rule of thumb that assumes £20/kWp per year which includes a fund to replace the inverters every five to 10 years.

At Fairglen low energy housing project the residents are responsible for maintenance. The customer manual recommends that PV panels are cleaned every six months, and a full electrical inspection carried out every 10 years.

Inverters installed under PV panels

Lessons learned

• Factor in adequate metering from the outset to maintain performance and to claim the feed-in tariff.

• Commissioning is quick, but some post-commissioning monitoring is necessary. The output should be checked at least once a month.

• Consider who is responsible for ongoing cleaning and maintenance.

• Make sure facilities management is trained on how the system should operate and perform.

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17Photovoltaics

Hackney Academy, London

London School of Hygiene and Tropical Medicine

Stoke Local Service Centre, Stoke-on-Trent

Fair Glen, Phase 1 (two houses)

One Brighton

Description of project

New build school academy

Refurbishment of Grade II listed university building

New build extension to community building,

New build residential houses

New build residential apartments

Rating of PV array 21.1 7.4 8.3 2.4 9.36

Area of PV array (m2)

175 57 59.4 55 68

Predicted energy output kWh/yr

15,000 5,750 7,930 1,915 5,400

Monitored energy output kWh/yr

11,641 2,500 6,936 2,101 10,122

Monitored kWh/kWp

552 338 836 875 1,081

Cost per kW £6,445 £33,496 £6,265 £6,940 £6,944

Total cost (supply, installation, testing and commissioning)

£136,000 £247,867 £52,000 £16,655 £65,000

Project summaries

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CIBSE Understanding Building Photovoltaics,

2000. Intended to provide guidance for engineers and other building professionals on the design of PV in buildings.

Capturing Solar Energy, 2009. An overview of the available solar system solutions, technologies and applications for buildings. It considers design and installation issues as well as commissioning and maintenance requirements.

Carbon Trust Feed-in Tariffs Policy and Markets Guide.

Information for organisations interested in applying for feed-in tariffs

Department of Trade and Industry

Photovoltaics in buildings. Guide to the installation of PV systems. 2nd edition, 2006. Intended to assist supply system installers to ensure that mains-connected PV systems meet UK standards and best practice recommendations.

Further information

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CTG038

The Carbon Trust receives funding from Government including the Department of Energy and Climate Change, the Department for Transport, the Scottish Government, the Welsh Assembly Government and Invest Northern Ireland.

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Published in the UK: March 2011.

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