Off Grid Power Systems II – Generating Power

In part one I wrote about some practical properties of electricity and energy, in this part I write briefly about realistic ways of generating electricity for off grid home use.  Focusing on costs of setups and on quantifying how much power is generated. Part III will be about storing electricity, maintaining batteries, charge controllers and DC-AC inverters.

This is a comment I added to the previous post (Off Grid Power Systems I – Electricity):

  • To gain a better understanding of quantities like power, voltage, current, charge, etc., it is helpful to consider the analogy to flowing water.

There are basically five realistic ways to generate electricity:

  1. Photovoltaics: a method of generating electrical power by converting solar radiation into direct current electricity.
  2. Solar Thermal Energy: harnessing solar energy for thermal energy (heat). Think of solar cookers and solar hot water generation. This heat can be converted into DC electricity. However, efficiency is bad and for normal households this is not a viable way to generate electricity.
  3. Windpower: conversion of wind energy into useful forms of energy, such as for instance creating electrical power or pumping water.
  4. Hydropower: same as wind power, but derived from falling and running water.
  5. Fossil fuel burning generators: is the combination of an electrical generator and and an engine

Let’s start with 5), petrol and diesel fuel burning generators in terms of pros and cons. Well, generators require only a small amount of space, they are quite reliable and as long as you top up the fuel they keep running. Units producing 2-5 kVA (remember Volts times Ampere yield Watts, so that is 2000 – 5000 W continuously) are quite affordable. Diesel generators will be more expensive but in general they require less fuel per hour and diesel seems to be cheaper than petrol in many countries.

However, they are loud and produce exhaust fumes, so you will need to dedicate a generator shed, never put them underneath your house as many like to do here in NZ. You cannot see all the toxic fumes that an engine produces and if for whatever reason it catches fire and you installed it underneath your house, well, don’t be surprised when all turns into ash.

Have a look at my home-made diesel generator which puts out about 1000 W and which I never use; home-made diesel generator. While I don’t want to write any more about engine generators, obviously they are a good backup, especially if you live in a larger household and run freezers, fridges and so on. I would argue, however, that you only need them for backup. Many will say that they need a generator because they run it at night for only a few hours. “It’s just a few litres of diesel per day.”, is what I hear all the time.

Well, buddy, if you do this for a few years and consider maintenance costs as well, and also the fact that it is a dirty and loud way to generate electricity and has nothing to do with green energy, you’ll find that it is a better option to save money for a few years and then invest into a decent, green system. Seriously, people install these generators here on the island, so they can run a washing machine and what not. Once you’ve got one, it is very hard to get away from it, because you can produce large amounts of electricity by burning diesel. You can do this whenever you want.

I find it highly amusing to see all the residential homes here on the barrier ‘on the main road’. Properties with less than 1000 square meter land, where your neighbour is a stone’s throw away. You can just imagine how much it must piss you off as a property owner if you invested in green and quiet ways of generating electricity and your neighbour just turns on a diesel generator at night…. ‘Tack, tack, tack, tack, tack…..’ Every night….

4) Hydropower is non-arguably the best way to generate off grid electricity. Unfortunately, hydropower is often overlooked, although it should be clear that if you are living next to a permanent stream, you have a permanent 24/7 source of power.

To estimate how much power you can generate, you only need to work out two quantities. i) the flow of water and ii) the height between inlet and outlet. In order to figure out the former, you can work out how long it takes to fill up a 20 l bucket. To work out the latter can be a bit tricky, especially if the height difference is more than 10 m. An easy way to estimate this is using an altimeter.

To work out how much power you could produce via the stream or river, you need to write down the following:

b = density of water in [kg/m^3]. Since the density is rather constant we’ll use 998 kg/m^3.

c = flow of water in [m^3/s]. I’ve got here at the house site 100 ml per second which is equivalent to 0.0001 m^3/s.

d = acceleration due to gravity. Another constant at 9.8 [m/s^2].

e = head, or height difference between inlet and outlet in [m], I’m estimating mine at 40 m.

a = is an efficiency coefficient of the turbine, it will be dimensionless (no units). Obviously, if the water is directed from the stream to the hydro-generator via pipes, there are other things to consider, like resistance for instance, but these ‘losses’ can be incorporated into this efficiency coefficient.

P=a*b*c*d*e is all we need to do. Let’s do a calculation using a flow rate of 0.0001 cubic meters per second and a head of 40 m.

P = a * 998 * 0.0001 * 9.8 * 40 * kg/m^3 * m^3/s * m/s^2 * m = a * 40 * kg * m^2/s^3 = a * 40 * W

[Watt = J/s = (kg*m^2/s^2)/s]

So what did we get? 40 W continuously with an efficiency of 100 %. That is almost 1 kWh per day or in other words, it is about 3 A @ 12 V continuously.  It is not much but will be enough to run LED lights 24/7 or a laptop for half a day.

If you followed the calculation, you’ll notice that it is mainly about the flow rate, but there are hydrogenerators that are optimized toward flow rate and some toward head. 100 ml per second is not much, my creek will do about 1 litre/s after hard rain, although that won’t change the flow rate at the end of my pipe outlet. In theory though, by utilizing larger diameter pipe, I’d be at 400 W at 100 % efficiency.

Another important point to stress is that the hydro generator should be as close as possible to your battery bank and home. As mentioned earlier, the total resistance of wires increase dramatically with length for DC electricity. An alternative would be to convert DC into AC at the hydro power source.

There are some good online resources about hydro power, you might want to check out the Harris Hydroelectric Systems with a Pelton-type runner and a home-made alternative using a 20 l bucket.

Hydropower is – at the moment – not an option for me, since the creek is some 100 m away from my dwelling.

3) Windpower

In principle, power is generated by a wind generator similar to a hydro generator and I like to point out a few important factors when considering what unit to buy.

Wind is a great alternative energy source and a great complement to photovoltaics, since it is often windy when it is cloudy. I think that you want to buy the biggest unit you can afford, forget a 200 W unit, go for 1000 W or at least 500+ W. The only way to get proper information about wind generators is to read the manufacturer’s specification sheet. This will enable you to make the right call in terms of buying a cheap unit or a more expensive unit.

  • Noise level is an important factor. Although it is usually loud when it is windy, you still want to buy a unit that is designed to work quietly. Often that means more than three blades or a larger blade length.
  • The minimal wind speed at which the generator starts to produce a significant amount of power is arguably the most important factor. I’ve seen data sheets of cheap Chinese units that required 12 m/s of wind to start producing power. That is a wind speed of 43.2 km/h. That means this unit won’t do anything in a breeze.
  • The maximum wind speed it can handle is also important. Good units will have a brake system, which reduces the rotations per minute in storms and also when your battery bank is fully charged.
  • In analogy to hydropower, the wind generator should be close to your house, which is sometimes just not that feasible. It will be a trade-off between noise level and of course you want to install it where it is most susceptible to wind, but your house is built where you are best sheltered from wind. A good unit will have an inverter build in. The inverter ‘transforms’ DC electricity – which the wind generator puts out – into AC electricity. As mentioned before, it will cost you much  more to transport DC electricity 50 m compared to AC.
  • Wind generators are more or less maintenance free, but when something goes wrong, you want to be able to access it easily. Stationary tripods are thus not a good idea.

2) Solar thermal energy. As mentioned, transforming heat into electricity is in most cases not a viable method for personal off grid power generation, although it is a very neat way for large scale systems. However, it should be used where possible to heat water up.

Commercial solar water heating systems have been on the market for a long time, soon I will make my own by running some 50 m of black alcathene pipe on the roof which connects to an insulated water reservoir.

Further, I’m also really keen to experiment with melting salts via solar radiation. The idea is to focus solar radiation onto a reservoir containing a solid salt, thus melting it. In analogy to pocket heaters, solar energy is used to melt the salt and when this is enticed to solidify again, it will release that energy in form of heat.

1) Photovoltaics

Solar panels are getting cheaper and cheaper. A 200 W 24 V unit cost about 320 NZD. As mentioned before, get 24 V panels even if you run a 12 V system. You just need to buy the right charge controller – I will discuss charge controllers and DC – AC inverters in the following part about storing energy. Obviously, solar panels must be aligned according to maximum sun exposure. Here, on the southern hemisphere, you want them to point north. However, in order to maximize efficiency, the panel array would be mounted in such a way that both horizontal and vertical axes of individual panels or the entire array can be altered according to time of day and time of year.

This is often not a viable option for individual households and you are left with two options. i) mount them on the house roof or ii) on a pole.

The former has the advantage that the array does not require any specific space, further, it is secure from high winds. Solar panels don’t need maintenance other than wiping them clean (this can be difficult on the roof). Alternatively, you mount them on a pole. This would be preferred if your roof is not facing north or if you want to alter the angle of the panels toward the sun. However, you must consider high winds.

Photovoltaics is a quiet way to generate electricity and with life expectancies of physical structure and power output of 25 years (80% guaranteed by manufacturers) and more, it pays to invest a bit more right from the start.

In the next part, I’ll write about storing DC electricity, battery maintenance, charge controllers and DC-AC inverters.

4 thoughts on “Off Grid Power Systems II – Generating Power”

  1. Hey mate, nice article!
    I’m not an expert in this, but as far as I know most generators produce AC power, due to the rotating magnetic field… most likely every 12V generator has some AC/DC converter build in (cheapest way: 4 diodes and some capacitor to stabilise the output, some migth also “switch” the direction mechanically, like a DC motor does, with a commutator) because it is most convenient to use with batteries and all the other equipment. The AC output frequency is not constant, but related to the rotational frequency of the wind/water generator. I never thought about it, but it might be more efficient to ret rid of the build in AC->DC converter, transfer the AC current straigth from the generator itself over longer distances and use the converter (extracted from the original generator “assembly”) just next to your batteries. I’m not sure though, how much the efficiency of AC transfer depends on the frequency …. but maybe worth a try. Converting ac(generator itself)->dc(inside generator)->ac(before transmitting)->dc(charge batteries)[->ac(your 230V or whatever house supply)] sounds like loosing a HUGE amount of the power produced in the generator itself.
    Cheers,
    Urban

    1. Thanks Urban, I’ve taken a couple alternators apart and as you say, most generators produce AC. I’ve got to do a some reading on different types of generators though before I can properly comment on this (thinking here, permanent magnet, induced magnetic field generators, dynamo, etc.). Indeed, AC-DC-AC-DC and then back to AC for 230V home appliances is not the way to go. So yeah, that’s me for now in regard to this, but I’ll get back to it one of these days. A few things happening here at the moment, which is good. Cheers buddy, B

  2. Hi Ben,
    Really interesting to have found your articles. I’m interested in understanding what coastal, non grid electricity costs. For instance, wee islands/communities that solely use diesel generators for all power requirement – what does it cost? i.e including diesel transport costs for refuelling etc. If I’m going to present a renewable energy resource as a viable alternative solution I need to be able to demonstrate positive differences in costs. Have you seen any credible sources with this sort of data anywhere on your travels?
    Many Thanks, Jenn

    1. Dear Jenn,

      thanks for your comment. Have a look at http://cleantechnica.com/, you might find what your are looking for. It certainly will be a non-trivial task to compare the overall price per watt for renewable/alternative energy sources with for instance solely using diesel generators, as many variables need to be considered that have a mid-/long term price tag which are not easy to identify.

      http://thinkprogress.org/climate/2013/08/22/2508191/germany-solar-generation-record/; I believe that one misses the picture by focusing on the costs of renewable energy sources.

      As an example, it might be cheaper to operate a fossil burning plant to provide an off-grid community with power, but that community might experience
      a growth in tourism, marketing, etc. by doing the same with renewable energy sources.

      Cheers, Ben

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