Probably the most common use case for off-grid lighting is in outdoor areas that need low-intensity security lights, signage or marker lights.
According to Johnathan Rowles of Dragons Breath Solar, these might include “car parks, walkways, moorings and marinas, skate parks and other locations where it is needed to prevent unsociable behaviour and keep security aspects under control”.
His firm, among others, supplies integrated solar-powered lights for these applications. He says: “We have now manufactured solar lighting systems for most of the UK’s councils over the last 20 years, never having had any issues with those products.
“No two applications are the same in the UK, so we have to take a brief of what the customer is trying to achieve and build a design around that specification. Otherwise all we are doing is selling something that may not cover the scope in time and power required.
“Solar lighting has to be dynamic enough to capture all of the available sunlight on a given day. Usually the winter is when it is needed to counteract the short daylight times and long evenings.
“Mid-winter in the UK can be 17 hours of night, so you only have the dependency of five possible hours of daylight to harvest the charge capacity of batteries, so we size the equipment to match the customers’ needs.”
INTERIOR LIGHTING
Interior lighting is where there is a real challenge: the intensity of lighting is likely to be much higher, so the power requirements are greater. Firms such as Photonic Universe produce complete kits of PV cells, inverter, charger, battery and lights, which will provide lighting for a small off-grid building, such as a shed or workshop, for a part of the day. A small ‘25W’ kit, which includes four 5W 12V lamps, costs less than £150 all in, and can power its lights for six to seven hours per day. According to marketing manager Sophia Highland, customers vary widely, from agricultural users to leisure vehicle owners and holiday yurt operators.
Other applications for these off-grid power systems can include CCTV and communications equipment. For these more ambitious schemes, Photonic Universe has also just introduced larger-scale off-grid kits operating at 48V: its 3.6kW kit includes 12 300W monocrystalline solar panels, each around 1.0 x 1.6m in size. The panels feed an inverter/charger rated at 5,000W, which can be monitored via Bluetooth; this charges a bank of 24 AGM lead-acid batteries with a total capacity of 24kWh. The inverter/charger can also drive 230V mains appliances directly, and even cope with the starting current of an electric motor. This kit (without any lighting components) costs just under £6,000.
VEHICLE-TO-GRID
Some people will already have a substantial storage battery available, in the form of an electric car. Vehicle-to-grid (V2G) systems have been in place in Scandinavia for a while, and have been trialled in the UK by Nissan: the electric vehicle (EV) is charged up either by a local microgeneration source, such as PV or a wind turbine, or from the grid when electricity is cheaper and/or more likely to have been produced by renewable sources.
The vehicle’s battery can then return electricity to the grid at peak times, potentially returning a net profit. However, since the Feed-in Tariff (FIT) scheme closed to new applicants earlier this year, this is no longer tempting. Off the grid, that is clearly not relevant, but a parked EV could provide electricity to an off-grid building on a small scale – some of the latest models have a capacity of up to 100kWh.
Can you make solar-powered lighting pay? Bristol-based Solarsense believes so: it designed and installed a system for Versa Engineering’s workshop building, with funding from an asset finance provider over five years at a low interest rate. The loan was entirely offset by the savings on electricity bills and FIT payments. While the FIT payments are no longer available to new customers, the system does have a projected lifecycle of 25 years – and will save around 294 tonnes of CO2 emissions over that time.
BOX OUT: How much power can you get from a solar panel?
Solar panels are typically rated in terms of kWp – but this is the peak power (in kilowatts) they can deliver. As you can imagine, this is not always achieved in the UK, and the exact location can make a good deal of difference: “If you offered something to somebody in Southampton it probably wouldn’t work for somebody in the north of Scotland,” says Johnathan Rowles of Dragons Breath Solar.
The positioning and attitude of the solar array is critical, and seasonal factors need to be taken into account. The basic performance of the solar array is calculated by multiplying the power rating of the PV cells (kWp) by a shading factor (SF) and a site correction factor (KK): kWh = kWp x SF x KK
The site correction factor KK comes from a table based on the location of the array, with corrections according to its orientation and inclination. An example of the difference between actual and rated energy output comes from the Versa Engineering installation (see main text): here, 115 panels each rated at 260W give a total potential output of 29.9kWp. The estimated annual output of the system is 28,920kWh – representing about 967 hours per year (or 2.6 hours per day) of that rated output.
BOX OUT 2: How much light do you need?
According to the HSE document ‘Lighting at Work’ (HSG38) (www.is.gd/iyodef), the recommended lighting levels for different tasks range from 20 lux (for the movement of people or machines, such as in a corridor) to 500 lux (for work requiring perception of fine detail, such as in a drawing office or electronic assembly plant). Lux is the measure of ‘illuminance’, or the amount of light output (in lumens) that falls on a square meter of surface. And these are average values, with permissible minimum values between a quarter and a half of these numbers.
As an example, a typical 20ft x 8ft (6.1 x 2.4m) portable building has an internal floor area of around 14.4m2. To illuminate this to an average level of 200 lux (as recommended by the HSE for offices) would require 14.4 x 200 = 2,880 lumens (lm) of lighting.
How much power would this require? That depends on the ‘luminous efficacy’ of the lighting, in terms of lumens per Watt (LPW or lm/W) of input power. While the very best new bulbs can reportedly produce 200LPW, a more realistic target is around 120LPW for a T8-style LED tube fitting – this is around twice as efficient as a fluorescent or CFL tube. So for our case study we would require 2,880/120 = 24W of LED lighting.
This is a rough estimate, as there are other variables at work: the efficiency of a light depends on the type of fitting, its colour temperature and its CRI (colour rendering index – a measure of how accurately the true colours of things can be seen). So it is best to get lighting advice from a professional.