Jun 042013
 
The fickleness of wind and other renewable energy sources means you need to store power as best you can when it is available, for future use when it is not available.

The fickleness of wind and other renewable energy sources means you need to store power as best you can when it is available, for future use when it is not available.

This is the third part of a four-part article series about how to store electricity (better thought of as storing energy rather than electricity per se).  If you arrived directly here from a link or search engine, you might wish to start from the first part of the series here, then read on sequentially through the balance of the series.

Flowing Water Uphill and Other Gravitational Energy Stores

Another interesting approach to storing electricity is by pumping water uphill.

If you have a hill on your property and sufficient water, you can use surplus energy to pump water up to a large reservoir on the top of the hill, and then when you need electricity, you run the water down the hill and to a hydro-electric plant at the bottom.

This can store energy for a longer term – the only storage loss being evaporation – but involves potentially massive amounts of water and as much vertical rise/fall as possible.  If you double the height differential, you halve the amount of water you need, and vice versa.

Although this might seem like an unusual and strange technology, 93% of all stored power, world-wide, is stored this way.

To put the amount of water needed into perspective, if you wanted to store enough water to be able to generate 5kW for 100 hours – a good reserve for emergencies) you could either have an efficient diesel generator and about 50 gallons of diesel, or if you were able to store water somewhere 100 ft higher than ground level, you’d want to have about three million gallons of water in the reservoir.  That is about 400,000 cu ft of water, so think of a pond measuring perhaps 200 ft by 200 ft and 10 ft deep.  You’d also need another storage area at the bottom to hold the water prior to pumping it back uphill again.

This is of course not impossible, assuming you have a way to get the vertical rise that is necessary, and a source of water to replenish losses from evaporation.

The concept can be extended and slightly modified, indeed, it is merely an extension of the concept found on many cuckoo clocks, where a weight slowly descends, driving the clock mechanism, then is ‘rewound’ when you use energy to pull the weight back up again.  Anything that uses gravity as a way of storing energy can work.

There is a project currently being developed in California that uses rail boxcars loaded with heavy gravel.  Electric locomotives use spare grid power to haul the boxcars up a length of steeply inclined track, and then when the electricity is needed to be returned to the grid, the boxcars are allowed to push the loco down the track, and the loco now acts as a generator feeding power back into the grid.

Such a system can be 80% or more efficient, and will store energy indefinitely with no storage loss.  The boxcar example requires more power to push the boxcars uphill than would be available from a typical private power source, but this could of course be modified (lighter boxcars and/or less steep grade).

These types of systems can require a fair measure of space.  In the water example, if the water reservoirs at top and bottom are ten feet deep, you need almost 2 acres of space just to store the water, plus more space for the pipes and generator/pump.

But their biggest requirement is the height differential.  If you’ve got a fully flat property, you’re either going to have to create a raised portion, which would involve a prohibitive amount of earth working, or else look for other alternatives, which presently tend to invariably circle back to batteries.

Using a Flywheel to Store Electricity

Flywheel technology is becoming more practical, although still a technology that is equal parts experimental and/or not ideally suited for our applications.

With a flywheel, spare electrical energy is used to spin up the flywheel – a huge heavy massive device that spins rapidly – and then when electrical energy is needed, the flywheel is used to run a generator.

Flywheels can store surprisingly large amounts of power, and if in a near vacuum and with magnetic bearings, are slow to lose their energy (by ‘slow’ we mean the energy loss rate is acceptable for a device that charges during the day and discharges at night, but not quite so acceptable if you want to be able to store energy for several days).  They can provide a reasonably efficient means of storing power.

They are also fairly low maintenance, especially if kept in a semi-sealed environment.  But they are also large heavy devices, potentially weighing 5 – 10 tons or more, and require very precise balancing and bearings due to the speeds they spin at.

Flywheels are best suited for applications that require large amounts of sudden energy and/or applications that have large amounts of energy suddenly surplus.  That’s not to say they’re not potentially good for our sorts of applications too, and the good news is that the growth in renewable energy generation is feeding growth in related issues, particularly energy storage.

We expect to see small-sized flywheel installations continue to be developed to a point where they may become practical for storing energy for short-term overnight use, but we’ve not encountered a flywheel that is quite ready for prime time just yet, alas, although there are some hopeful developments underway at present.

There are many other technologies that are either in a development stage, or which are not suited for our sort of scale of energy storage – for example, compressed air.

How Much Electricity Storage Do You Need

Storing electricity is a somewhat wasteful and somewhat expensive process, although it could be convincingly argued that having extra spare electricity being unused at certain times of day to also be wasteful too!

Some electricity storage is both prudent and essential.  The question becomes – how much.

If your only renewable energy source is PV, then you need enough for two purposes :

First, you need enough to get you through a typical night from when the energy flow from the cells diminishes as the sun gets low until such time as the energy flow restarts when the sun rises the next morning.

Second, you need additional ‘spare’ capacity to cover times when the weather is bad, the sky is full of clouds, and the PV cells aren’t generating enough electricity during a normal day to replenish the store for the next night.  Indeed, in a worst case scenario, the PV cells might not even provide enough power to cover the normal needs during the day.

You can of course control this to a certain extent by increasing the capacity of your bank of solar cells.  If you need 20 kWhrs of electricity a day – 10 kWhr for daytime use and 10 kWhr to store for the night, then you’re less likely to run into shortage with a setup that is rated to provide 40 kWhr per typical day than with a setup that is rated to provide only 20 kWhr per typical day.

So the greater your surplus during a typical day, the less an amount of reserve storage you need.  If even a cloudy day sees your PV installation providing enough power for the day and the night, all you would seem to need is a single night’s worth of power to be stored.

We’d still like to increase that capacity to allow for mishaps and emergencies.  What happens if something fails in the solar setup and it takes you a day or more to repair/replace it?  Also, if you only need to recharge your batteries every other day rather than every day, clearly you’ll get twice the life from them, and if you are normally only discharging your batteries down to 50% full, you’ll again get much longer life than if every night you are discharging them down to 20% full.

If you are only using wind power, the first thing we’d recommend would be to rush out and buy some solar cells!  Some days there might be no wind, but some sun (and vice versa); by spreading your electricity generation between two different sources, you are reducing your risks and increasing your resilience.

Beyond that, it becomes harder to predict how much wind power you can expect.  You need to look at detailed wind logs for your location, and approximating the above ground height your turbine will be situated at, so as to get a feeling for average, best case and worst case daily and nightly power generation.

Using a process a bit like that we recommend for working out your water needs, you can proceed to calculate some likely pessimistic scenarios about the amount of wind power you might get, and from that, you can then work out how much stored electricity you’ll need.  Don’t forget to allow for an unexpected several day outage occurring at exactly the worst possible moment, too!

But, wait – before you do these sums, you should first re-examine the question of how much electricity you will need and use, and reduce this amount as much as possible.  The next three sections cover this concept.

Continued in Part Four

Please now click on to read the final part of this series, ‘Strategies to Reduce Your Need to Store Electricity‘.  If you’ve not yet read them, you might also want to read the first two parts of the series too – Storing Electricity and Using Batteries to Store Electricity.

We also have other articles on the general topic of Energy.

Jun 042013
 
Using solar water heating brings two benefits - greater efficiency and an easy way to store energy as hot water for later use when the sun goes down.

Using solar water heating brings two benefits – greater efficiency and an easy way to store energy as hot water for later use when the sun goes down.

This is the final part of a four-part article series about how to store electricity (better thought of as storing energy rather than electricity per se).  If you arrived directly here from a link or search engine, you might wish to start from the first part of the series here, then read on sequentially through the balance of the series.

A Different Sort of Energy Storage – Time-shifting Electricity Demand

Storing electricity is usually both the clumsiest and costliest way of productively using up spare/surplus electricity.  You lose some electricity when you convert it to however it is stored, you lose more during the storage process, and then you lose even more when you convert it back to electricity again.

So it is better to time-shift as much as possible to times when you have spare electricity.  One example of a time-shiftable process would be washing clothes and dishes (assuming you still will use an electric dishwasher in the future, which is unlikely).

With these types of activities, you can wait until a day with lots of electricity coming from the PV cells or wind turbine before washing your clothes (and/or dishes).

If you have an electric range, you should use it to cook food during times of peak electricity production.  Maybe you’ll change your eating habits and have your main meal of the day in the middle of the day (something many societies already do and which many health experts think to be a better approach to eating) so that the cooking energy is sourced when the sun is shining brightly or the wind blowing strongly.

If you are vacuuming, do that when there’s spare electricity, and never when you’re using stored electricity.  There’s almost never an emergency requiring urgent vacuuming in the middle of the night!

Try and match your own sleep and wake times to the sun.  If you’re sleeping during the morning with daylight flooding in to the retreat, but then staying up late at night, using electricity to power lights and possibly heating too, that is more wasteful than using the sun for light as much as possible, and sleeping in a warm bed during the coldest part of the night.

Take your shower/bath in the morning if possible so as to allow the system all day to reheat the water.  Or, alternatively, after using hot water at night, don’t use stored electricity to heat up the replacement water, and instead wait until the morning to do that.

Which leads to another strategy.

A Different Sort of Electricity Storage – Storing the Results of Using Electricity

Another approach to storing electricity is to store the results of using electricity.  We started to get into that in the preceding section – heat your hot water when you have spare electricity to do so, and then deplete it when you do not have spare electricity, and only reheat it again when you have the spare electricity back to do so.

An easy and low tech type of storage is to store heating or cooling energy.  There are special ‘storage’ heaters that will run when you have spare electricity and heat maybe a large vat of oil or even a block of concrete, then at night, you expose this heat source and allow the heat to transfer out and into the surrounding areas of your retreat.

You can also do this for cooling.  When there’s surplus electricity, make large blocks of ice.  Then, when you need extra cooling but no longer have extra electricity, allow the ice to melt, cooling the area around it.

These are low tech but sensible concepts.  Indeed, many large buildings currently use these types of systems for more efficient heating/cooling.  You can do the same.

Replacing Electricity With Other Energy Forms

Electricity is an ‘expensive’ form of energy – it is close to the top of the ‘energy pyramid’, just like beef is close to the top of the food pyramid (in terms of the amount of feed and water needed per pound of meat).

If you can replace electricity with other forms of energy – forms which may be more abundant, and/or lower tech and more sustainable into the indefinite future – you’ll be doing yourself a favor.

One example of this substitution is to mount a solar water heater on your roof.  That way, instead of using solar energy to create electricity and then the electricity to heat the water, you go directly from the solar energy to the hot water, in a much more productive/efficient process, and also in a lower-tech form.

A solar water heater is easy to construct and easy to maintain.  But as your PV cells fail, you’ll absolutely not be able to repair them yourself, and no-way can you fabricate your own additional solar cells in a low-tech future.

Another example is to use wood (or coal or peat) as a heat source for heating your retreat, your hot water, and possibly even for cooking with as well.

Or, if you’re using propane to power a generator, get a propane rather than electric stove and burn the propane in that stove.  You’ll use less propane to directly power your stove than to first convert it to electricity and then to second convert the electricity to heat on your stove.

How Much Extra Electricity Generation Capacity Do You Need

Remember that the first part of an electricity storage system is having ‘spare’ electricity to store!

Once you’ve done what you can to minimize your reliance on stored electricity, the next question becomes how you will get the electricity to store.

Clearly, you will do this by increasing the capacity of your renewable energy sources so they can simultaneously meet your normal electricity needs and also send extra capacity into the storage system you selected.

And, equally clearly, the electricity source(s) you have must be able to generate enough spare electricity each day to be stored to be used each night.

So you need to consider the answer to this question – ‘assuming a less than fully sunny day, or a day with poor rather than optimum wind, how much power do I need generated for the day’s requirements and to recharge for the night ahead as well?’.

Remember that most solar and wind generators are specified as having a certain capacity on a good/close to optimum day.  You’ll need to adjust the claimed daily capacity to more accurately reflect the real-world rather than best-case probability of electricity it will create for you.  Maybe you need to get a system which is apparently twice the capacity you need, just so that in periods of low generating conditions, it still generates enough for both your immediate and your storage needs too.

Now, for an interesting additional factor.  The greater your storage capacity, the smaller your surplus daily generating capacity needs to be.  This is because the extra storage capacity gives you a greater ability to average out the peaks and troughs of actual daily electricity production.

One more consideration.  Most lead-acid batteries require a certain minimum charge rate in order to effectively charge, so you need to be sure that your electricity source will deliver enough charge to actually load electricity into the battery.  Think of a helicopter – it uses a lot of energy just to hover – you don’t want your battery to just hover, you want its charge level to rise, so you need to provide more than this ‘hovering’ amount of energy to actually raise the battery’s charge.

Whatever you come up with, you must have a system that on an average/ordinary day will provide enough electricity during the day for all the daytime needs in your retreat, plus enough left over to replenish a typical night-time’s consumption of electricity, plus perhaps still more left over to replenish one more typical night-time’s consumption, too.

Electricity Will Never Be Cheaper Than Now

Spending $10,000 for a 5 kW solar array might seem like a lot of money, and of course, by any measure, it is indeed a substantial sum.

But – and you can absolutely trust us on this – in a Level 2 and particularly a Level 3 situation – your future lifestyle and ability to survive will be totally linked to your ability to generate your own energy, particularly in the form of electricity.

So get as much electricity generating capacity now as you can afford.  Particularly if it is modular, it can be either used or traded in the future, and you’ll quickly discover the essential nature of energy in a future Level 2/3 situation.

Read the Rest of the Series

If you’ve not yet read them, you might want to read the first three parts of this series too – Storing Electricity, Using Batteries to Store Electricity and Other Energy Storage Methods.

We also have other articles on the general topic of Energy.