PV-Wind-Diesel Hybrid power system

PV-Wind-Diesel Hybrid power system

Downsview Farm: Living off-grid in London, England

The horse stables roof had the best solar exposure

2 Kw of PV, 500 watts of wind power and inside, a 17 Kw diesal standby generator.

As an American, I used to think of London as a city which is thick with fog all the time. I lived there for three years, and it does have plenty of gloomy weather. When Brits asked me about my accent and I replied that I was from California, they would say, What are you doing living here, then? After telling them that I work for a solar company in London, they would just laugh and say good luck!

Imagine my surprise, then, to be called to visit a potential customer living only 12 miles from the heart of London, inside the M-25 beltway, who needed an off-grid alternative to replace his aging generator-only electrical power system. The farm is surrounded by large rolling meadows. His only source of power was an old, oil-blowing 30 kW diesel generator. London Electricity wanted £100,000 ($160,000) to bring in AC lines across a mile or two of a neighbours field.

The farms owner, David Burgess, wanted to replace the 30 kW generator set with reliable and quiet solar electricty. Plus, he was adding a new building, requiring a new electrical service. The large main house had an attractive traditional English tile roof, which could not be altered, and the new addition was too shaded for PV. The old unused horse stables, however, were sorely in need of a new roof, and had good southern exposure.

Robert Warren at Downsview Farm

A 30 kilowatt Mastervolt Powercenter is the key to a safe and reliable system.

Finding a suitable solution to a large off-grid home

I offered him a solar solution that works well in cloudy and overcast weather: a Unisolar roofing product in the form of 17 watt PV shingles. Due to the low efficiency (6%) of these amorphous type modules, a greater roofing surface area is required. On the plus side, this triple-junction technology can deliver about 10 to 20% more power per installed kilowatt over a years time than a similar sized set of crystalline PV modules. They can turn the blue part of the light spectrum into power.

Another factor in choosing the size and type of this PV system was the fact that in England, permits are difficult to obtain from local authorities, if historical buildings undergo any renovation that alters the overall appearance. So we couldnt install PV modules. These old stables needed a new roof anyway, so I measured up the two roofs available, and had room for 72 of the 17-watt shingles on each of two buildings. Standard 3-tab composite shingles were used on border and valley areas, while the solar shingles were grouped into two sets of 9 strings of 8 modules in series, for 1224 watts peak per building.

I chose to use a 48-volt battery bank, and wired the flexible Unisolar shingles in series sets of 8 to produce an optimum charging voltage of 58+ VDC, from the 2.4 kW of PV. This also simplified the installation process, as one Trace C-40amp/48V controller could handle each building.

I usually prefer to use wet cell, deep cycle batteries in off-grid systems, but as London doesnt have freezing problems, and the customer preferred maintenance-free batteries, so we chose 48 Yuasa 2-volt sealed batteries, rated at 320 A/Hrs. each, for 30 kW/hrs of useable storage, about two to three times his daily requirement. This could have been a larger battery bank, but then small PV system wouldnt be as effective in topping them off. (When sizing the batteries, I look closely at the manufacturers specification for available current with battery discharging limited to not below 1.8 volts per cell. Then I set the charge controllers settings and auto-start feature for the generator accordingly, so that once this voltage is ) The batteries are rack-mounted in their own a separate room, directly behind the wall where the inverters are placed.

He needed a new diesel generator, as the old one was on its last legs. I chose a 15.6 kW Lister-Petter, made here in the UK, with a good reputation for reliability. Sizing this home power system (see sizing worksheet) was at best an educated guess, with a who is customer is not fully aware of his energy needs. It had to handle heavy-use periods such as the Christmas season, when the weather is horrible for days on end. So in winter months, the generator is a primary source of power, while in summer months it runs a lot less. I needed inverters with a good surge capacity.

So I ordered two Trace SW4548E inverters and connected them in parallel. This way, the diesel generator could efficiently power both battery chargers integral to the Trace inverters, charging the 48-volt, 30-kilowatt/hour battery bank. While just one 4.5 kW inverter may have done the job of running household loads for a good part of the time, we needed two inverters for efficient battery charging and in case all household loads were turned on at the same time. Plus, the built-in generator logic functions of the Trace inverters provided a reliable method of keeping tabs on the batteries and keeping them happy.

There was also a good potential for wind energy to supplement the PV array, as the house is on a small hill. The problem with installing wind systems in the UK is that zoning prohibits towers over 10 meters (30 feet). So I ordered an Air-403 wind generator, which we securely bolted on a mast with custom brackets on the West end of the stables. I had these prefabricated and hot-dip galvanized, so the wind generator mast is held firmly by steel plates sandwiching both sides of the concrete block wall. A taller freestanding mast, though superior, would have required endless hassles to get a permit, even though the farm is not visible from neighbouring houses or roads.

The bottom line for Mr. Burgess is that at the end of one year with a PV system, his fuel bill has been cut to less than 20% of what it used to be. It runs for around 4 hours/day instead of 16 hours, and he has all the power he needs. The two Trace inverters with their good surge capacity and their ability to provide additional surge capacity to the generator output provide very close to a 30kW surge capacity.

David Burgess told me, after my last visit, if you ever need any statement or recommendation for solar energy, I would be quite happy to give you my highest endorsement! This system has brought us out of the dark ages into the 21^st Century!

How the wind, PV, and diesal work together