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Dryden Gait, 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 three blocks of flats at Dryden Gait, to generate electricity for the communal building needs.

The three buildings at Dryden Gait were constructed in the 1980’s and are very similar in terms of roof dimensions and orientation. 

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 Dryden Gait buildings 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

There were no available energy bills for the communal energy consumption at Dryden Gait, so this has been estimated to be 4,000kWh per quarter.

The azimuth of the roofs is -40° due southeast, the roof pitch is 25° and the useful area for each building is 109m2.

The options examined for the roofs at Dryden Gait are shown in the table below.

 

Option 1

Option 2

Option 3

Solar Panel Technology

Poly- crystalline

Mono- crystalline

Mono- crystalline

Phases

Single

Single

Single

Number of Modules

42

42

42

Power (wp) per Module

230

230

235

Total Power (kW)

9.7

9.7

9.9

Number of Invertors

1

1

1

Size

9.0kWh

9.0kWh

9.0kWh

Strings

3 x 14

3 x 14

3 x 14

Produced Energy (kWh/yr)

7,600

7,385

7,688

Specific Production (kWh/kWp/yr)

787

765

779

 

The proposed solution is option 3, which has the highest efficiency and greatest annual production.

Lessons

Option 3 would have a capital cost of £30,000. With a FIT rate of 37.8p, the system would be paid back within 8 years. A total income and profit of more than £150,000 could be achieved over a 25 year lifetime, and 95 tonnes of CO2 could also be saved.

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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