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Duff Street, 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 two apartment buildings in Duff Street, to generate electricity for the communal building needs.

The two buildings on Duff Street have a total of 26 flats. They were built in 1993, and are part of a cluster of similar buildings, with parking and grass areas in the middle. The roofs have a metal profiled sheet finish on insulation batts and timber deck, supported by mono-pitch timber trusses.

The average annual electricity consumption in the common areas of building one was 3,900kWh and in building two was 2,100kWh.

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 Duff Street 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

The two building are orientated -20° (southeast) but the roofs have low pitch at 10°. It is recommended therefore to increase the pitch with racks to an optimum 30°.

Building one has a useable roof area of 70m2, while building two has a useable roof area of 140m2. There are no issues with potential shading of PV panels.

The options examined for the roofs at Duff Street are shown in the table below.

 

Option 1

Option 2

Option 3

Solar Panel Technology

Polycrystalline

Monocrystalline

Monocrystalline

Phases

Single

Single

Single

Number of Modules

42

51

39

Power (wp) per Module

230

185

250

Total Power (kW)

9.7

9.4

9.8

Number of Invertors

1

1

1

Size

9.0kW

9.0kW

9.0kW

Strings

3 x 14

3 x 17

3 x 17

Produced Energy (kWh/yr)

7,420

7,507

7,598

Specific Production (kWh/kWp/yr)

768

796

779

Lessons

Option 3 was selected as the most suitable option as it provided a greater output and also incurred lower capital costs. The total energy produced by the PV system (7,600kWh) would provide in excess of that required for the communal building needs (approx. 6,000kWh) meaning that around 20% of the energy produced could be sold back to the grid.

The capital cost of the system is around £30,000, which would be paid back within eight years, when considering the cost of maintenance, the savings on energy and the income from FIT. Additionally, exporting around 20% of the energy produced to the grid could generate around £50 per year initially, rising to £150 per year.

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 Duff Street 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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Case Study

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