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St Colms, 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 sheltered housing accommodation at St Colms, to generate electricity for the communal building needs.

The buildings at St Colms have a variety of apartments for both sheltered housing and rented accommodation. There is also a Church at ground level. Two roof areas have been analysed; one above the main block of flats (PV1) and another above the Church (PV2).

Electricity consumption in the common areas is generally stable for much of the year (around 11,500kWh per quarter), with an increase in the winter months (to 13,000kWh) due to increased lighting needs.

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 St Colms 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 details of roof one are:

Orientation:  -15° (southeast)

Pitch: 25°

Area: 55m2

 

and roof two:

Orientation: 50° (southeast)

Pitch: 25°

Area: 79m2

 

There is the possibility of the Church clock tower shading the roof of PV2, so this has been considered when determining the useful area.

The options examined for the roofs at St Colms are shown in the table below.

 

PV1

PV2

 

Option 1

Option 2

Option 3

Option 1

Option 2

Option 3

Solar Panel Technology

Mono- crys- talline

Poly- crys- talline

Mono- crys- talline

Poly- crys- talline

Poly- crys- talline

Poly- crys- talline

Phases

Single

Single

Single

Single

Single

Single

Number of Modules

11

10

10

14

14

12

Power (wp) per Module

185

190

195

220

225

230

Total Power (kW)

2.04

1.9

2

3.1

3.1

2.76

Number of Invertors

1

1

1

1

1

1

Size

2.0kWh

2.0kW

2.0kW

2.75kWh

2.75kWh

2.75kWh

Strings

1 x 11

1 x 10

1 x 10

1 x 14

1 x 14

1 x 12

Produced Energy (kWh/yr)

1,595

1,475

1,540

2,194

2,300

2,018

Specific Production (kWh/ kWp/yr)

784

776

790

712

730

731

 

For PV1, the preferred system is option 3, which would produce 1,504kWh per year. For the larger PV2, the preferred option is option 2, which would produce approximately 2,300kWh per year.

Lessons

The chosen options for both roofs would allow for a 43.3p FIT to be collected. The payback for PV1 would be within 12 years – this includes capital and maintenance costs, FIT gains and savings on grid electricity. CO2 savings over a 25 year period are estimated at 18.9 tonnes. The payback for PV2 would be in 11 years, again considering all costs and savings.

 

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 St Colms 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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