Micro-generation

I am intrigued by the concept of domestic energy self-sufficiency, however, it is far from clear how this might be achieved in south-east England.  In an attempt to understand what might be possible and the limitations of a given scheme, I have secured a 4.7w solar panel to a pole and taped it to a railing and declared it a research project.  So far, no answers, but things which are being thought are discussed below.  The exercise started in November 2007 so that some experience could be gained during the winter months.  Energy generation and consumption is a seasonal activity and to make some form of informed decision it is necessary to see how a proposed scheme behaves over a full year.  Also, it should fit the lifestyle of the people involved.

How

The wind turbine is the icon of alternative energy, whilst large (>1 MW) units perform well in rural and marine locations, small ones (<2.5kw) in urban environments have limitations.  Most small wind turbines appear to need a constant wind speed of 10 m/s (approx. 20 mph) to generate significant amounts of electricity.  In our location, the number of days in the year when the wind speed is greater than 5m/s is too few to be worth the effort of counting them.  Thus solar panels appear to be the only viable option (with caveats).

I would like to learn more about small scale heat and power systems which provide both electricity and heating.  These use some form of fuel to power a electrical generator, the waste heat from the engine is used for space heating and hot water supply.

Relationship with the National Grid

From the small number of schemes I have read about and seen, solar panels are typically used to power an inverter (a device which takes 12V DC as input and outputs 240 V AC), the output from which is fed to a two way meter.  When the sun shines, surplus electricity is sold to the national grid, at night or when a big cloud passes overhead, the house buys its energy from the fossil fuel/nuclear generated supplier on the national grid.  A typical 2 - 3 kw solar installation might generate 4,000 kwh over a year, matching the consumption of an energy conscious household and making them carbon neutral.  However, at a guess 75% of the energy is generated between 08:00 and 16:00 in the interval between the solar equinoxes in March and September.  At other times, the household is as dependent on the national grid as the rest of us.

An alternative configuration is to use some form of storage device (e.g. lead acid batteries) as a buffer.  Thus energy generated during the day is available to power lighting etc. at night.  I started this exercise in November 2007 by monitoring a small 1.5w panel through to January 2008, to my surprise, it did generate measurable amounts of energy for between two and four hours/day when the sky was clear of low, thick cloud.  One of the objectives of this project is to understand how a solar panel functions in winter and its ability to maintain one or more lead acid accumulators and what level of support is required from the national grid.  Total self-sufficiency is not going to happen, but it may be possible to reduce dependency on it.

How the Energy is Used

There are two aspects to the use of energy from micro-generation.  The first is the form in which it is consumed.  Whilst electricity arrives as a standard 240 V 50 hz mains supply, in our house, few devices use it in this form, most, eg laptop computers, Freeview box etc. convert it to DC between 3 and 19V.  So conversion losses are significant (and hard to measure).  At first sight, opting for a 12V household main, seems like a good idea, however, low voltage systems create high currents and these are potentially dangerous.  Secondly, there is a wide range of voltages and some form of shift up or down would be needed.  A practical and safe solution is to stick with 240 V AC, thus an inverter has to be built into the system.

The peak loads imposed by devices should be considered, the two devices in our household which impose high instantaneous loads are the laser printer and the washing machine, neither use outrageous amounts of energy, but the laser printer peaks at 750w during the fusing process and the washing machine gets up to 2,500w during the spin part of its cycle.  It is probably not desirable to design a system to cope with high peak loads, it is better to opt for machines with a manageable peak load, such as an inkjet printer and some form of low-tech washing machine that takes the drudgery out of laundry without the need to build a new fossil fuel power station.

LED Lighting is a technology to watch as it is efficient and operates at low voltages.

Cost

By opting for for micro-generation, you are paying for your electricity in advance, whilst it is reasonable to assume that energy price inflation will be above the overall rate for the foreseeable future and an individual may want to pay a premium to reduce their carbon footprint, any project should make some form of economic sense.  One way of determining a budget is to calculate the NPV based on your current electricity bill, say for a ten year period.  Thus if your electricity bill is £350/year, using a discount rate of 7.5% suggests that your budget is around £2,500.  Depending how you view the future of energy prices and carbon reduction, this might stretch to £5,000.  This figure is considerably lower than the £10,000 to £15,000 cost of some recent installations.  Thus a micro-generation project is an economic and technical challenge.

Page Updated: 14-Mar-07

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