Well-positioned for optimum performance

Because the system performance depends on the solar collector being able to take advantage of available sunlight, the collector’s physical location is crucial in determining efficiency. However, the collector does not need to be facing due south in order to work effectively.

The map on the right and the table below provide an approximate guide to the technical performance of the system. As with all solar systems, there are a number of factors that influence both its ability to produce solar thermal energy and its overall efficiency.

Solar thermal energy efficiency depends on geographical location, polar orientation and pitch of the roof. The system’s overall effectiveness will also be influenced by the efficiency of the auxiliary heating source, and the household’s daily rate of hot water consumption.

For simplicity, we have selected four cities to represent the different meteorological zones on the map. Using the stated solar irradiation at these points we are able to calculate the savings achieved by a typical VELUX solar hot water system.

Performance of a typical VELUX solar hot water system

For the purposes of demonstrating performance, VELUX has split the country into four regional areas: North (Glasgow), Mid North (Manchester), Mid South(Cambridge) and South (Southampton) so that a reasonable comparison can be made.

Annual solar irradiation in the 4 regional areas

Region 1 North Glasgow 1032 kWh/m2
Region 2 Mid North Manchester 1109 kWh/m2
Region 3 Mid South Cambridge 1160 kWh/m2
Region 4 South Southampton 1286 kWh/m2

Performance of a typical system
Using the above information, and following a recent study that showed the typical consumption of daily hot water (DHW) per household in the UK is between 30 and 50 litres, at a temperature of 55°C per person, the total benefit of the VELUX solar hot water system per region is:

Region 1 North Glasgow 2681 kWh/year
Region 2 Mid North Manchester 2874 kWh/year
Region 3 Mid South Cambridge 3003 kWh/year
Region 4 South Southampton 3222 kWh/year

A typical VELUX solar hot water system: four person household (5m2 of solar collectors 280 litre tank) due south orientation with 80% efficient gas boiler.

It is essential to consider the following when designing a solar hot water system
• Occupancy level
• Geographical area
• Polar orientation
• Pitch of roof
• Efficiency of auxiliary heating
• Size of collector area

Annual solar irradiation on 40° pitch, facing due South

Orientation of the collectors

The following tables demonstrate the performance of the VELUX solar hot water system in the four chosen regions. Each table shows the direction of the roof and its pitch (or inclination). These tables show that solar energy is a viable option as a renewable energy source in the UK.

In new build cases, these factors can be taken into consideration at design stage and the orientation and design of the roof pitch can be adjusted for optimum efficiency. In renovation projects, the data serves as a guide to calculating the approximate savings available.

For example, in region 1, a collector facing South at a pitch of 40° will achieve 100% performance. However a collector facing WSW at a pitch of 30° will achieve 88% efficiency.

Area Factor

In order to maximise the solar irradiation available when it is not possible to achieve due south orientation and 40° pitch, it is possible to increase the collector area to compensate for this. The following table shows the relevant average multiplication factor for the collector area.

In most cases, the variety of standard VELUX solar hot water systems are sufficient for the majority of design applications. However, should you wish to design a bespoke system please contact VELUX solar energy. 

Energy efficient, cost effective

We have completed three case studies which give an indication of the likely value of the energy savings that can be achieved by the installation of a VELUX solar hot water system. For the purposes of these case studies, we have assumed that the auxiliary boiler is gas fired in all cases.

Case study 1
This case study evaluates three collectors with a combined area of 7.5m2 and a 375 litre tank, typically specified for households of between four to seven occupants.

3 x collectors at 7.5m2 with 375 litre tank

Summary:
The solar fraction that can be gained increases from a significant 59% in region 1 to a maximum of 73% in region 4. The solar benefit that can be achieved with a 70% efficient boiler within region 4 is 5835 kWh/year.

Case study 2
The following table displays the results of a solar hot water system designed for a household of three to five people, with a 280 litre tank, and two collectors that give a combined area of 5m2.

2 x collectors at 5m2with 280 litre tank

Summary:
In region 4 a valuable solar fraction of up to 67% can be gained. In addition, when combined with a 70% efficient boiler a productive solar benefit of 4032 kWh/year can be achieved.

Case study 3
This case study looks at a typical solar hot water system designed to suit two to three people, using a 180 litre hot water storage tank, with one 2.5m2 collector.

1 x collector at 2.5m2 with 180 litre tank

Summary:
This combination can create an advantageous solar benefit of up to 2075 kWh/year when combined with a 70% efficient boiler.  The maximum solar fraction created supplies a productive 52.2%.