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Cathay Court, Edinburgh

Solar PV feasibility study for homes in the City of Edinburgh

Overview

Cathay Court is a four storey 'T' shape building, built in 1995. It is a sheltered housing complex, with 33 self-contained apartments for couples or single people.

This study identifies the feasibility of installing photovoltaic (PV) panels on the roof of Cathay Court, to generate electricity for the communal building needs which include a lift, laundry facilities, lounge and lighting.

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. Cathay Court was selected due to its 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 document).  

 

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 energy consumption in the apartments is paid directly by residents. Malcolm Homes provided consumption data for the communal areas. Average monthly consumption (across a year) was 3,059kWh, with an average payment of £278 each month. Total annual consumption was just less than 37,000kWh.

The roof selected for the PV panel installation is orientated 10° South East, and is pitched approximately 35° from the horizontal. This roof has approximately 220 m2 of useable space – excluding the intersection area in the centre of the 'T'. This intersection splits the roof into two spaces, approximately 94.5m2 and 77m2. Neither of these roof areas are shaded.

The options examined for this roof at Cathay Court are shown in the table below.

 

Option 1

Option 2

Option 3

Solar Panel Technology

Polycrystalline

Monocrystalline

Polycrystalline

Phases

Single

Single

Single

Number of Modules

51

42

42

Power (wp) per Module

190

235

230

Total Power (kW)

9.7

9.9

9.7

Number of Invertors

1

1

1

Size

9.0kW

9.0kW

9.0kW

Strings

3 x 17

3 x 14

3 x 14

Produced Energy (kWh/yr)

7,848

7,920

7,837

Specific Production (kWh/kWp/yr)

810

802

811

 

In order to achieve the best FIT rate, it was important to keep the system below 10kW power output.

Lessons

Option 2 was considered for further economic analysis. With an estimated annual output of 7,920kWh, the system would be capable of contributing up to 21% of the annual communal building demand (of 37,000kWh). This would indicate a saving of around £1,000 per year.

The estimated cost of option 2 is £30,574.06. With a FIT rate of 12p per kWh and including expected maintenance costs, the system would achieve payback within eight years.

 

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 Cathay Court 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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