Fact Sheet March Solar Photovoltaic Systems Electricity from Sunshine

Transcription

1 Fact Sheet March 2013 Solar Photovoltaic Systems Electricity from Sunshine

2 Farming connect Introduction Farming has always been about harvesting energy from the sun after all, every green leaf is a solar collector that stores energy from sunlight. But today solar photovoltaic (PV) systems allow us to directly convert sunlight into electricity. With rising electricity prices and increasing concerns about the contribution of coal and gas power stations to climate change, the idea of tapping the sun for clean, green electricity has never been more popular. And with access to large roof areas and open fields, farmers are in an ideal position to harvest solar electricity. Electricity or hot water? There are two fundamentally different types of solar panels: those producing hot water, known as solar thermal systems, and those for producing electricity, solar photovoltaic (PV) systems. A solar thermal system consists of either glass tubes (evacuated tube system) or a box with a flat glass sheet (flat plate system that looks a bit like a roof window) mounted either onto or directly into the roof. A solar PV system consists of thinner panels, mounted either onto a roof on rails, with a gap between the roof and the panels or free standing in a field. This fact sheet provides information about systems producing electricity from sunshine using PV panels. 1

3 Solar Photovoltaic Systems Electricity from Sunshine How does solar PV work? A typical solar panel, also known as a module, consists of a number of cells made from very pure silicon the same material that computer chips are made of. When sunlight falls onto these cells it causes an electric current flow across the cell. A module consists of a number of these cells each typically the size of a beer mat connected together, with a protective glass sheet in front and a protective sheet at the back and, sometimes, with an aluminium frame around. Modules come in many different sizes; many systems installed today use modules with a rated capacity (see below) of around 200 Watts these are approx. 1.5m x 1m in size. A small domestic scheme might consist of only 10 of these modules whereas at a large solar farm there could be several thousand. As the PV modules produce direct current (DC) electricity, an inverter unit is required to convert to alternating (AC) current of mains electricity. In a small domestic scheme, the inverter is little bigger than a shoe box in a larger system it is the size of a wardrobe

4 Farming connect How much electricity can it produce? Understanding rated power PV modules are sold with a rated power which describes how much electrical power it can produce under certain standardised conditions. As the power will very rarely exceed the rated value, this can be understood as the maximum (or peak ) power output capacity. Therefore, a solar PV system consisting of 50 modules of a rated capacity of 200 Watts has a rated peak capacity of 10,000 Watts (10kW), sometimes referred to as 10kW(p). However, the amount of electricity produced at any given moment will depend on the amount of light that falls onto the modules. Solar panels can produce electricity even on a cloudy day or when they are shaded or facing away from the sun. But the yield will be much higher when the modules are oriented directly facing the sun. As a result, the amount of electricity produced will not only depend on the size of the system but also on the location (in Wales, the South receives more solar energy than the North) and on the orientation of the panels. Roof orientation and energy yield To produce the maximum amount of electricity, a solar module would follow the sun as it rises in the east and sets in the west. Such mechanical tracker systems do exist but they are not very common in the UK as they require more space and as the costs often outweigh the benefit. Most UK systems have a fixed orientation and tilt. The most common arrangement is that modules are mounted flat onto a roof, though larger solar farms using fixed ground-mounted frames have become more popular with farmers. The ideal configuration for a system with fixed orientation in Wales is to face south with a tilt slope of around 35 - this ensures that the modules face directly towards the sun when its strongest, in the middle of a summer day. 3

5 Solar Photovoltaic Systems Electricity from Sunshine Figure 1: Influence of orientation on solar yield, relative to the optimal orientation (facing 0 due south, 35 tilt) Figure 1 shows how the annual energy yield from a solar roof changes with tilt and orientation. For example, a 10kW PV roof mounted on a perfectly south-facing, unshaded roof with 35 slope in Builth Wells will produce around 8,800kWh per year (880kWh/kW/yr). On a south-west facing roof with a 15 slope the yield is approximately 10% lower and hence the 10kW roof will produce around 8,000kWh per year (800kWh/kW/yr), and a west facing roof with a 40 slope will produce around 7,000kWh/yr, around 20% less than the ideal orientation

6 Farming connect Is my roof suitable? There are a number of factors that determine whether a roof is suitable for a solar PV system: Orientation As previously explained, south facing is ideal but even roofs facing due east or west can be financially viable. Roof slopes are usually not a problem but if the slope is not steep enough (less than ) then the rain may not be able to wash off enough dirt, creating a need for more frequent cleaning. Size/area An area of 6-8m 2 is required per kw of module capacity, so a 4kW roof will occupy around 30m 2 of roof space. Structural strength PV modules are usually mounted on a roof using rails anchored in the roof under-structure. Modules are not very heavyaround 12kg/m2 but with older buildings the strength of the roof could be an issue, especially where strong winds can exert forces on the panels. Roof material Some roof types make mounting modules easier than others. The corrugated metal sheets of modern agricultural barns are relatively unproblematic. Old slate roofs require experienced roofers to minimise the risk of damage or leaks. Work on asbestos roofs will involve strict health & safety precautions. Shading Objects casting shade onto PV modules anything from a hill or tree on the horizon to a chimney on the roof can have a negative impact on the performance of a solar roof. Due to the way modules are wired up, even a relatively small shaded area can cause a significant reduction in energy output. If there is any risk of shading, then make sure the installer takes this into consideration. In addition, you will also need to think about: Electricity connection The electricity connection to your farm needs to be strong enough to accept the electricity produced by your PV roof. Systems up to 4kW (or 12kW on a three phase connection) can be installed without prior permission from your distribution network operator (DNO), usually Western Power or Scottish Power. Planning permission A domestic solar PV system may not require planning permission if it is not on a listed building and does not protrude from the roof. Non-domestic PV schemes will usually require planning permission. 5

7 Solar Photovoltaic Systems Electricity from Sunshine Economics Cost The cost of a fully installed solar PV system depends on a number of factors and may also change over time. In early 2013, the typical cost of a 4kW system was 6,000-8,000 whereas a 10kW system would typically cost 12,000-15,000. Income An agricultural business installing a PV system will benefit financially from: Generation Tariff (also known as Feed-in Tariff, FiT) payments for every unit of electricity generated, whether it is used at the farm or exported. The current generation tariffs are given in Table 1. Export Tariff payments of 4.64p/kWh for every unit of electricity exported to the electricity grid (i.e. not used at the farm). Electricity Savings for every kwh of solar electricity which is used directly at the farm (rather than exported to the grid). The value of electricity savings depends on the rate paid for electricity and on how much electricity is used. Table 1: PV Generation tariffs as of May 2013 Description Rate p/kwh 0-4kW >4-10kW >10-50kW >50-100kW > kW > kW >250kW-5MW 6.85 Stand-alone 6.85 Generation tariff rates for new installations are reviewed every quarter and depending on uptake of the technology over the preceding months, tariffs will be decreased by 0%-28%. However, it is important to stress that any changes will only affect systems installed after the changes take effect. The tariff valid on the day a system was commissioned is the tariff which the system will receive for all electricity it produces over the 20 year tariff lifetime. The only changes which will affect a system once it has been commissioned is an annual adjustment of tariffs linked to the retail price index (RPI) in other words, generation and export income will increase with inflation

8 Farming connect Eligibility for FiT income Since April 2012, an Energy Performance Certificate (EPC) level D or better is required for the building to which a solar PV installation is attached or wired to provide electricity or otherwise only the lowest generation tariff will be paid. To achieve EPC level D, energy efficiency measures such as wall and loft insulation and double glazed windows are required. In general, energy efficiency measures of this type are highly recommended as a very cost-effective way to reduce energy costs. However, in the case of some Welsh farm buildings, e.g. listed buildings with solid stone walls, it can be difficult to meet the required EPC level. There may be exceptions to the EPC requirement for PV systems mounted on agricultural barns if they are connected to a separate electricity connection. Also, to be eligible for FiT income, the system will need to be installed by a professional accredited under the Microgeneration Certification Scheme (MCS) see Finding an installer overleaf. Example calculations Table 2 below illustrates some example financial calculations for small-scale PV systems of 4kW and 10kW capacity two common sizes for roof-mounted systems. Table 2: Example financial calculations System size 4kW 10kW Roof area required (approx.) 30 m 2 (300 sqft) 75 m 2 (750 sqft) Solar yield 3,400 kwh/yr 8,500 kwh/yr Capital cost (incl. installation) 7,000 14,000 Operation & maintenance, per year Generation tariff income, per year 525 1,189 Export tariff income, per year Electricity savings, per year Total benefit, per year 779 1,938 Payback time 8 years 6 years Assumptions: The calculations above assume solar yields of 850 kwh per kw capacity per year, typical for a mostly south-facing, unshaded location in mid-wales. The figure for electricity savings assume that 50% of the electricity is used locally, replacing electricity purchased at a rate of 15p/kWh. 7

9 Solar Photovoltaic Systems Electricity from Sunshine Next steps Estimating solar yields For an unshaded PV system, it is not too difficult to estimate the amount of energy that can be produced in an average year. There are a few free online tools (see below) which can help you. However, these tools will not be able to estimate the impact of shading. In cases where this could be a serious issue, it is recommended that installers or consultants should use specialist software to simulate the effect of shading on electricity production. Finding an installer The Microgeneration Certification Scheme s website (see overleaf) has a list of all MCS accredited installers; it is easy to search by postcode to find professionals in your area. It is advised that you get quotes from several companies. Ideally, the installer should have experience with your type of roof, especially in the case of slate roofs

10 Farming connect Resources Contact Finding installers & consultants Microgeneration Certification Scheme installers database: Microgeneration Certification Scheme installers database: Tools for estimating solar yields Centre for Alternative Technology quick & simple Energy Saving Trust quick & simple Calculator Photovoltaic Geographical Information System advanced For more information on Farming Connect services and events, contact us: Phone menterabusnes.co.uk website /farmingconnect Catch up with all the latest news from Farming Connect by joining our online community. Follow us on: Facebook facebook.com/farmingconnect Farming Connect support, guidance and training is delivered by Menter a Busnes on behalf of the Welsh Government Author: Tobi Kellner, Centre for Alternative Technology 9