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Firhill, Oxgangs, Edinburgh

Solar PV feasibility study for homes in the City of Edinburgh

Overview

This study identifies the feasibility of installing photovoltaic (PV) panels on the roofs of residential buildings at Firhill, to generate electricity.

The Firhill complex has a mix of 4-storey apartment blocks and terraced houses, forming a courtyard in the centre. The properties were built in 2003, and have timber, slate finished roofs.  

Approach

The Scottish Energy Centre, part of the Institute for Sustainable Construction at Edinburgh Napier University were selected by CIC Start Online on behalf of Malcolm Homes Ltd to conduct the feasibility study for the installation of PV panels at their properties in Edinburgh. The buildings at Firhill were selected due to their optimum roof shape and orientation. Both technical and economic feasibility have been considered. The approach taken in the study was as follows:

  • Discuss and evaluate the technical constraints of the building and site, and demonstrate best practice in terms of location, orientation and azimuth – in the Northern hemisphere, PV panels should ideally face between south-east and south-west, in Scotland the most efficient tilt angle is ±2° of 40°. Panels should also be located so that they are not shaded throughout the day.
  • Discuss and identify potential constraints of the energy suppliers requirements – as the UK electricity network has traditionally had centralised generators, distributing energy to domestic and commercial customers.
  • Identify the FIT (feed in tariff) benefits that may be gained – FIT are dependent on installation size (kWp) and rates are set by OFGEM for generation and export.
  • Identify the economic payback of equipment – based on capital cost, maintenance requirements, energy produced and FIT available
  • Explore the rent a roof scheme – where roof space is leased to allow a PV installation, where the householder gains the electricity produced and the installer the FIT (full details on page 27 of the case study).  

 

A design tool, PVSyst, was used to establish the economic and technical feasibility of PV installations. The tool needs the following information to obtain accurate results:

  • Tilt of pitched roof
  • Orientation of roof
  • The amount of output power required
  • The type of PV panels to be used

 

Other considerations include potential shading of the PV panels, FIT restraints and possible issues with grid connections.

A full explanation of how solar PV panels work is included on page 7 of the pdf download (see right).

Performance

Not all the roofs at Firhill were appropriate for PV installations, but three terraces were chosen for further analysis. These were selected as they had good orientation, access and available roof areas. 

There were no available energy bills for the properties at Firhill, so these have been estimated from industry benchmarks. All the terraced properties are two-storey, three bedroomed homes, with gas for space heating and hot water. The annual electricity consumption has been estimated at 4,500kWh.

Terrace 1 has an orientation of 10° due west, roof pitch of 35° and average roof area (per property) of 18.9m2. Terrace 2 also has an orientation of 10° due west (except one property which is due south) and roof pitch of 35° but the average roof size is bigger, at 24.2m2. Terrace 3 is orientated -10° due east, roof pitch is 35° and roof area is 18.9m2 on average.

The properties which are directly next to the apartment blocks are shaded at some points during the day and as such have not been considered suitable for PV installation. In total, 15 properties across the three terraces have been analysed. Full details of the proposed PV installation are shown in the case study document, available to download on the right.

Detailed analysis of two options for the roofs at Firhill is shown in the table below.

 

Property 14

(Terrace 1)

Property 41

(Terrace 2)

Solar Panel Technology

Polycrystalline

Polycrystalline

Phases

Single

Single

Number of Modules

9

12

Power (wp) per Module

230

230

Total Power (kW)

2.07

2.76

Number of Invertors

1

1

Size

1.95kWh

2.2kWh

Strings

1 x 9

1 x 12

Produced Energy (kWh/yr)

1,608

2,071

Specific Production (kWh/kWp/yr)

777

751

Lessons

Although the roofs at Firhill varied in terms of orientation and useable area, the proposed systems were very similar as small changes didn’t affect dramatically affect the output. All the proposed PV systems would be eligible for a 43.3p FIT rate. As such, the payback for the first system (Property 14) would be 11 years, with a total profit over 25 years of £25,000. Property 41 had a larger roof, and as such although the initial cost may be higher, the returns would also be greater and payback was also within 11 years. Profit achieved over 25 years would be £35,000.

Before considering a renewable energy installation, it is imperative that the building is energy efficient, either through recent construction to current building standards or retrofit of an older building to meet these same performance levels. Otherwise, poor energy performance will counteract the effectiveness of the renewable technologies. It was understood before the feasibility study that the Dryden Gait buildings met these requirements. 

A structural analysis should also be undertaken before installing PV panels to ensure that the roof can withstand the additional loads.

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