Tuesday, 23 October 2012

Going off-grid... Part 1 ... Musings on the possibilities....

Firstly, my apologies for what is quite a long post without any pictures to break up the text....one of the advantages of writing this blog is that, for potential major projects like this one, it focuses my ideas and also forms a written Design Basis & Facilities Description for me to refer to in the future, and is a basis for comparison after completion....

I think it would be very good for the soul if we could be totally independent of our electricity supplier and run the entire house off-grid.

How can this be achieved ?  Let's look at the possibilities....

Energy Demand

At present, we consume around 4,500 kWhr of electrical energy per year.  With a bit of economising, we may be able to reduce this by 10% or so but then we still need to find 4,000 kWhr.

For heating, I suppose we are already 'off-grid' in a way with the heating oil, in that we're contracted to no-one and buy the stuff only when we need it and from whoever is selling it cheapest at the time.

But let's throw the heating energy into the mix, too.   We used around 2,500 litres of oil last year.  For fag-packet calculations, let's say Kerosene 28 has a gross heat value of 10 kWhr per litre, i.e. we consumed 25,000 kWhr in heating oil energy.  With our boiler efficiency at say 80%, that's 20,000 kWhr of usable heat we pumped out into the house.

(...I've another idea to make a Stirling engine that sits over the boiler flue outlet and drives a generator, to recover some of this lost 20% of boiler efficiency.  I've already started on an outline design, but that's for a future post...)

So, the combined annual net energy requirements are 24,000 kWhr for both heating and electricity.

Power Demand

OK, so we've looked at total energy.  Now let's look at the maximum rate at which we need to draw-down this energy, i.e. the power requirements.

For electricity, the single largest power consumer in the house by far is the cooker.   This is directly connected to the distribution board via  a 45A cable and switch, so let's say 10 kW instantaneous demand if the ovens and hobs are all blazing away together (it happens maybe once a year, at Christmas !).   The next largest consumer in the house which is used on a very regular basis is probably the washing machine, at say 2.5 kW, or some of my power tools.

Let's ignore the 10kW electrical cooker demand for the present, since it's so far out of kilter with any of the other individual electricity consumers.  We have a large-ish combination microwave oven unit that's a grill, fan oven and microwave all in one, so maybe that could take care of 90% of our cooking requirements.  It's rated at 1,850W, 1,250W and 900W respectively for the oven, grill and microwave.  We could probably get rid of the large cooker and replace it with a more energy-efficient hob for cooking in pans etc, or simply just not use the oven in the cooker we have.

So, for design purpose, let's say that around 5 kW is the maximum instantaneous electricity demand, i.e. we have the washing machine on at the same time as the microwave in oven mode, the TV's on in the background, computers are working and it's dark so all the outside lights are on...

For heating, our total house internal volume is around 260 cubic metres – I've done that calculation before from my CAD drawings of the house.  As a rough rule of thumb, 1 kW of heating power is required for each 14 cubic metres, therefore the power required is around 18 kW.    This seems to correlate well with the fact we have an oil boiler rated at 15/19, i.e. a heat output range of 15kW to 19kW.

Electrical Energy Storage

If we're going to go off-grid, we'll need to have a means of storing energy, particularly electrical energy, no matter how it's generated. (We'll leave storing heat until later, since we may be able to come up with an on-demand heating method.)

The only realistic answer is a battery bank.  With 12V leisure batteries, a 5kW power demand will pull around 420A, which means some pretty hefty cabling to connect them up.   Maybe we can stack the batteries in pairs, or even fours, to make a 24V or 48V system.

We'll also need a pure sine wave inverter – I've seen a few of them on eBay rated at 3kW continuous with up to 9kW for short durations, so it's technically possible.

Let's go back to our energy demand, 4,000 kWhr per year is 11 kWhr per day on average.  To account for seasonal differences, let's say the daily design basis should be 15 kWhr.

With 12V batteries, this is a daily energy drain of 1,250 Ah (amp hours).  Maybe a bank of 8 x 225Ah batteries would be a start, but at 5 kW demand each battery would be pushing out more than 50 A....

Electrical Energy Generation

Now we have to think about putting the energy back into the batteries.   If we're drawing down 1,250 Ah per day, then with a recharging efficiency of say two-thirds (66.7%) we're going to need to put back 1,825 Ah per day, every day.

If we consider PV solar generation, we know that in mid-winter we're only going to see 7 hours of daylight, and sometimes we might not see the sun at all for several days.   Similarly, our experience with the experimental wind turbine means that we cannot depend on a consistent and predictable output even with a larger unit.

Therefore, any solar or wind array needs a 100% supply back-up system from a diesel generator or similar.  So, in our thought processes, let's ignore the PV and wind equipment entirely for now since anything that replaces or supplements the output of the generator is only a financial bonus – the generator itself is enough as the initial basis of system design.   

Assume we configure our generator engine with an automatic electric starter, and mount it within a sound-proofed enclosure so it's quiet enough to be running say 12 hours per day. 

At 12 hours running time, we'd need 1,875W electrical power generation and we'd be pushing around 160A back into the 12V battery bank.

So let's say we need a 3hp (2.25kW) engine driving three or four 65-75A car alternators.   As a more practical alternative, we could consider an old commercial 240V AC genset, which would provide the power directly in the form we need it most, i.e. for the house, and then simultaneously take an AC feed off the output for a large 12V DC battery charger.

We could even buy a brand-new 3kW diesel generator set with mains voltage output, and fitted with an electric starter, for around £500, but I'd be very reluctant to rely one of these cheaper units for over 4,000 hours of operation per year.

With generation inefficiencies, we'll assume that the maximum 2.25 kW of engine power is required continuously, i.e. 27 kWhr over the 12 hours.

Our diesel engine will likely operate at around 40% efficiency, i.e. it can convert 40% of the fuel energy to shaft energy, and therefore the fuel energy required is 67.5 kWhr per day.

At say 10 kWhr per litre energy available in heating oil (or similar), that's a fuel consumption of 6.75 litres per day.   We've done a bit of research and a diesel engine should run quite happily on a 4:1 or even 5:1 blend of heating oil and vegetable oil – the veggie oil also provides lubrication to the diesel injectors, since heating oil is nowhere near as 'slippery' as diesel.

Let's do some cash sums.  if we use domestic heating and vegetable oils to drive the genset, then say 65p per litre or £4.40 per day.    Our current annual electricity bill is around £700, and therefore we could run the genset on fuel alone for 159 days for about the same costs as now. 

So, whatever associated renewable source we choose needs to be able to meet the demand for the other 206 days if we're not going to be significantly out of pocket.....

Waste Heat from our Genset

Let's go back to our generator engine.  As we said, it's 40% efficient, and therefore rejecting 60% of the fuel energy as heat.  That's 3.375 kW of heat for every 2.25 kW of shaft power.

Ideally, we'd like to find a water-cooled diesel engine, but if necessary we could shroud an air-cooled engine and fit our own heat exchanger.

If we could collect 50% of this 'waste heat', i.e. around 1.7 kW, it could make a useful contribution to the house heating requirements.

For 12 hours per day, this is 20 kWhr of heat energy equivalent to over 2 litres of heating oil, or £1.30 per day.....

Combined Heat & Power

OK, so let's now turn the problem around slightly.  What if we size the system to heat our home entirely from the heat rejected by a larger engine and then consider the electrical generation as a useful by-product ?  A home-made diesel-engined combined heat & power (CHP) plant.

From earlier, say we need 20,000 kWhr of heat energy per year, at a maximum instantaneous power demand of 18kW.

If we first consider the 18kW to be 50% of the 60% rejected by an engine, if you see what I mean, then we need a engine rated at 24 kW shaft power or, to put it another way, which uses 60 kW of fuel power. 

If we run for say 12 hours per day, giving 720 kWhr, then this is 72 litres per day of fuel  !   And importantly, we must actually use 24 kW of shaft power, i.e. drive a load of sufficient size, to generate the heat rejection we require from the engine.    So, it's not an economic prospect.

Therefore, let's now look at an alternative system in which we generate less direct heat but more electricity than we need for the appliances, and then the use the available balance of the electrical energy to supplement the house heating requirements via electric oil-filled radiators.

I've done a few basic calculations and I reckon that generating around 9 kW of electrical power gives somewhere near the optimum fuel consumption to generate a balance of the electrical and heating energy available to draw from the engine. 

(...there's also another variation to consider, in that we could design the electrical side of the system on the basis of twice our demand, and take the next-door neighbour off-grid too – it's much more viable on a larger scale – it would increase the capital costs of the CHP installation by less than half, it would automatically generate more heat for us, and they could repay via regular contributions towards the increased running costs that would still be much lower than their current electricity bills...however, that's for a future discussion over several glasses of our home-made wine...)

We'd probably hook up an old 3kW 240V AC generator to the engine as well as several 12V alternators.  This would give us a direct 240V AC mains line and simultaneous battery charging capabilities without additional transformation and rectifying equipment.  It's likely the 240V AC generator shaft needs to be continuously rotating at 3,000 rpm (i.e. 50Hz),  to give the correct domestic frequency output, and the 12V alternators will need to run very much faster than this.  We'd probably run the 240V generator directly off the engine crankshaft and the alternators via step-up pulley systems, but these are relatively simple engine speed control and mechanical design issues.

In the winter, this engine is going to consume around 27 litres of fuel per day, so it initially appears to be quite costly...however, the CHP plant will only be started whenever there's a demand for heating in the house, and our associated renewable electrical energy installation will have zero fuel costs, of course.

Project Implementation

What we would need first then is an old diesel car engine, or even the whole car – the engine and gensets will be far too heavy to move around manually, and so having the equipment on a rolling chassis would make it easier to find a good location for the installation.  An existing enclosed engine bay would also be better for fitting more sound-proofing material.  Additionally, the car would make a secure and weatherproof enclosure for everything else – we could take out the seats and mount the fuel tank, batteries and inverters etc all inside the car space.

(...I'm not yet sure what the wife thinks about having a knackered old car shell sitting somewhere out in the garden in perpetuity, but we'll cross that particular bridge if and when we come to it....)

We'd also need to buy a standalone 3kW genset as a back-up for electrical energy when we need to maintain our main CHP plant (although regular maintenance would be scheduled for the summer months), or if it broke down in the winter. 

The fallback for heating in the event of such a failure would be the existing house oil boiler – we'd need some diverter valves in the plumbing to allow the house radiators to be fed from either the CHP plant or the boiler, or via both systems in tandem when the CHP is operating.

Renewables as an integral part of the installation ...

We would only start the CHP plant when we needed to heat the house, and so the intention would be to use wind or solar for the rest of the time.

Capital Costs...

All the equipment we'd need ain't going to come cheap, so I've done a quick-and-dirty guesstimate of most of the upfront costs I can identify for the CHP :-

£750, old diesel-engined car with a half-decent engine
£500, used heat exchangers, shrouds, ducting, pipework, insulation
£300, used or reconditioned generators / car alternators
£800, bank of eight high-capacity leisure batteries
£750, high power pure sine wave inverter
£500, back-up 3kW standalone genset
£300, cabling, switchgear, instrumentation etc
£200, plumbing modifications to the existing heating system
£100, bracket materials, drivebelts etc for the additional alternators
£100, additional sound insulation for the engine


plus, for an associated solar installation :-

£2,000, 3.5kW to 4kW of solar panels
£750, high-amperage solar charge controller
£200, materials to mount the panels
£250, cabling, switchgear, instrumentation etc


Everything we'd need would be bought secondhand in eBay auctions where possible.  I've assumed a solar installation for this initial estimate, but I reckon I could construct and erect a large wind turbine myself for around the same price in materials.  However, our experience with experimental installations of both shows that a solar installation is likely to be much more productive than wind at our particular location.

On the plus side, we already have a heating oil tank in the garden of 2,500 litres capacity, and we can pump from this into a 205 litre fuel drum in which the vegetable oil would also be mixed to feed the CHP engine.  We already have a few empty drums.

Operating Costs & Economics

Our current bills for heating oil and electricity are around £2,300 per year.  Let's assume that the CHP engine and supplementary boiler-firing are going to need roughly the same amount of fuel as we currently consume for heating oil (£1,600), so these costs would cancel each other out. 

Therefore, the installation has to make itself pay on the basis of zero external electricity costs, currently £700 per year.  Add in say £200 per year for equipment maintenance (we'd probably need one replacement battery every year), and that's a net saving of £500 per year.

So, if we could actually make the installation for our £7,500 budget estimate, then it's around a fifteen-year payback period. 

However, the cost of electricity will almost certainly continue to rise way above inflation as the constraints on UK generating capacity get tighter, and so maybe it's a better investment than it looks. 

Also, we might be happy to pay a premium just for the satisfaction of being off-grid...

And so the deliberations and researches will continue.....this would be a year-long project to fully implement, and therefore watch this space regularly for any further developments.

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