One of the biggest issues with wind and solar is that they are intermittent, and so either need storage or grid-tie to work. There really is no good storage, and generally storage-based systems are highly wasteful, throwing away most of the power you generate because you want to keep the storage near full. Grid-tie is the only green choice, but it’s expensive and requires expensive inverters and permits and more.
One solution to this to find work for your renewable energy source to do that fits well with its intermittent nature. Something that will take all the power you generate, but not mind if it comes and goes. Such loads are hard to find. One potential example is pumping water to filter a swimming pool. Its recommended to flow twice the volume of your pool every day in summer, which means around 10kwh of electricity with typical systems. Most people filter their pool using the same pump they use for vacuuming and pool maintenance, which is actually way more powerful than you need for filtering. They offer variable speed pumps, which use a low-power efficient speed for filtering and a high-power speed for vacuum and manual operations, and claim they save a lot.
For those who have a pool, the pump is using as much electricity as all their other appliances in some cases, and so it’s a win to make that greener. Unlike those appliances, the pool water can be filtered any time, as electricity is available, though you can’t let the pool go unfiltered for days, so it’s not perfect. For people who have time-of-use metering, they are wise if they only filter at night, and many do that.
The trick to perfect use of solar for pool pumping would be a smart, multi-speed pump able to run on both the DC from solar panels and the grid power. It would need to do the following:
- When there is power from the solar panel, run as fast as you can on that power, filtering.
- When you need high flow, switch to (or combine with) grid power for full power.
- Track the amount of water filtered, as well as temperature, and when the sun did not provide enough power, run the pump at night off grid power to make up the difference.
- For extra credit, have a sensor that detects how clear the water is, and adjust grid usage based on that, rather than just weather.
This system would make use of all the power from the panels. As a plus, you need more filtering in summer than you do in winter, which matches what panels do. However, you must not oversize your panels. They can’t be bigger than you need to do all your winter filtering on a series of sunny winter’s days, or you will be wasting their power then.
Key to this plan is that it’s easy to install. Put in the new pump and wire it up to panels. No inverters or electricians and perhaps not even any permits. It doesn’t feed power back to the grid or the house. This is key because panels are now getting very cheap (less than a dollar per watt) and as such installs and permits and other gear are more expensive than the panels.
There are some pool pumps with brushless DC motors sold for solar use. They are expensive and don’t do the smart tricks above, in particular using the grid to take up the slack. They depend instead on overprovisioned solar, or solar systems powering more than a pump.
For $700 you can also buy a floating solar pool filter. This is a nice trick because it’s self-contained, though it’s a rather large thing to float in your pool. It can’t handle the whole filtering load —in fact it only handles about 25% of the load of a typical pool and uses cartridge filters. As such, you still run the regular pump and filter on some schedule, you just run it a bit less.
I noted above that you can get variable speed pumps, and that these, it is claimed, us as little as 1/5th the energy of the full speed pumps for filtering. They cost 2-3x as much as basic one speed pumps, and as a result are not very common. This bodes poorly for the solar proposal here, because if customers aren’t willing to do the up-front investment to save energy for these pumps, few would do the added task of putting up a solar panel and plugging it into such a pump. Comparatively few, that is — solar nerds would love to do it.
As always, the best place to deploy panels to do this would be the sunny, coal-oriented regions like Arizona and New Mexico, where it turns out pools are pretty popular. Once again, the math says that if your goal is to use your money and time to make the world greener, it would be far better to get people in those places to install a system like this on their pools than for you to put panels on your house in California for anything. Putting panels up in California is something you do to feel good.
Another interesting alternative is wind. Pumping water with wind is perhaps the oldest wind technology out there. In this case, you might even be able to be like an old windmill, and be mechanical, by having the turbine drive a flexible shaft down to the ground to run the pump. Presumably some clever transmission would be needed to maintain filter pressure properly at all windspeeds. You could also do traditional electrical generation from the wind and power a pump like the one above.
Wind has its positives and negatives. Unlike solar, it does not have the natural higher capacity in the summer. It can be much more intermittent. Solar panels still do around 30% to 50% of their rated power on ordinary cloudy days (though this is quite variable based on the panels and local weather patterns) so there is pumping every day. Wind in most places comes and goes. At my house, the winds are high today but it would not generally be suitable as we go weeks without much wind. Wind also prefers a tower near the pool, which has many issues.