two-speed with solar panels

[semenzato]

Hi,

I just installed photovoltaic panels on the roof and had to switch to time-of-use metering. This is going to make my pool recirculation more expensive, since I also have solar pool panels and I need to run the pump at peak hours (summer afternoons).

So I am considering a two-speed pump, but I am not sure if it will work for me. The current motor rating is 3/4 HP. I get plenty of flow---my pool is small (about 10,000 gallons) and I only need to run the pump 5 hours/day or so. The pump is 8 or 9 ft below the pool water line. The solar panels are about 20 ft above the water line. I am positive I can get by with a lower flow, but I still need a lot of head.

Does anybody have advice for this situation?

Thanks!
Luigi

[cleancloths]

Well you should only need the head to initially lift the water to the roof. Once you have flow established the height of the roof has no bearing as you effectively are only lifting to the surface of the pool. You create a siphon pulling down from the roof which cancels out the lift.

As for time of day pricing you will probably find that if you put a cover on the pool at night you will have no need for the solar panels in the peak summer months and will be able to run the pump at night as you have done in the past.

[tphaggerty]

Why 2 speed in your case? Why not just wire in the pump so that it kicks on for solar demand, then kicks off when no-demand. You are probably going to want the full 3/4 hp to run solar anyway.

My solar controller has a relay for a booster (110V only, unfortunately) that could do easily handle the 3/4 hp pump (its a Goldline). If your pump is 220v, you could use that relay to trip a 220v relay to run the pump, or get a controller that has a built in 220v relay (I'm pretty sure some of the more expensive pumps do this).

Or, even cheaper, just set your pump timer to run for the 4 or 5 peak sun hours and then at night. No need for a relay!

[semenzato]

Thanks tphaggerty, good suggestions. I already run the pump with the basic Intermatic timer for the 5 peak sun hours---and that's enough. It's an indoor pool, it doesn't get much dirt.

With time-of-day metering, electricity at peak sun hours costs about 50c/kWh. The pump probably uses the full 3/4 hp, which is about 600W, so that's 3kWh/day and $45/month. There is enough sun here (Oakland, CA) to take advantage of solar heat May, June, July, August, September. You're right, maybe I shouldn't bother, but still. According to the literature, I can get 1/2 the flow for 1/4 of the power, so I would save around $150 each summer, plus about 1/2 of that in the winter.

Cleancloths: thanks for the comments. I thought of the siphon effect too. but then I wondered. There is a valve at the top of the panels that lets air in when the flow stops. I believe this is done so that the panels do not get scrunched by the vacuum produced by gravity pulling down the water. I would think this means that the water must still be under pressure when it gets to the top. Siphoning implies negative pressure, but the valve would not allow that (unless it allows SOME negative pressure---but can it be that smart?)

[cleancloths]

I don't know how the valve works so I cannot tell you. I don't think "scrunching" would be an issue, as they would be filled with water as would the pipe feeding them when the pump stops.

Why did you switch to time of day? What is the savings off-peak that makes it worthwhile? I have peak / off-peak here in NJ as well put the rates are something like 18 cents and 9 cents versus a flat rate of around 15 cents.

I did it because I have a 10KW PV array on my roof which generates about 12,000 kwh a year almost all during peak times. I always used to run my pump at night as I have a heatpump to heat the pool so it made sense. But after much thought I am now running the pump during the day as the heatpump is much more efficient when it is 80 degrees out then when it is 30-40 degrees at night. Also, I added a pair of 2' by 20' panels to supplement the heatpump so I need to run during the day for that too.

[mas985]

The solar valves release when there is a vacuum in the line without any pump pressure. However, what cleancloths was describing is simply negative head not a vacuum. The pipe will still be pressurized throughout the whole run. Head is added with a pipe rise but then is subtracted with a pipe fall. Net net is zero head gain due to the fact they are on the roof (the panels and pipe runs will add head). You just need the high speed to fill the pipes but after that, low speed is sufficient. So after priming, the total head loss for solar is the same if they were on the roof or on the ground, assuming the same lengths of pipe.

[semenzato]

I think I have a valve because after the pump stops I can hear water coming down from the panels for a minute or two. When the pump restarts, for the first couple of minutes only air flows down from the panels and into the pool through the jets---and there is no doubt about that! The explanation of why the valve is needed also makes sense, so I have never questioned this.

I switched to time-of-use because that was required by our power utility (PG&E) in order to sell them energy. We just installed 30 Sharp 170W PV panels (which by the way appear to produce a peak of 4300W, according to the inverter).

I like the cost of electric energy in NJ! But at that price I don't think I would have installed the panels.

[semenzato]

Is there really a difference between "negative head" and "vacuum"? As I see it, unless there is still positive head at the top of the panels, the water on its way down is pulled by gravity and produces a "suction" at the top. By suction I mean a pressure lower than atmospheric. If the system is sealed, that negative pressure "sucks" the water up from the inlet. But if there is a valve, the suction will make the valve open. As I said, it's possible that the valve will stay closed with a small amount of suction, but eventually it will open, and I would imagine the margin to be too small to be of use.

Thanks!
Luigi

[cleancloths]

I switched to time-of-use because that was required by our power utility (PG&E) in order to sell them energy. We just installed 30 Sharp 170W PV panels (which by the way appear to produce a peak of 4300W, according to the inverter).

I like the cost of electric energy in NJ! But at that price I don't think I would have installed the panels.

That is interesting, as I think I am the only one in NJ that has time-of-use with solar panels - they actually had a hard time getting my bill right for the first six months because of it. My system is comprised of 54 Sharp 185W panels and we get a similar yield to yours, the max I have seen on the inverters is about 8600W.

You probably would have done it in NJ even with our "low" electric rates. That is because the state has a very aggresive program to promote solar. At the time I installed mine they were offering rebates of up to 70% or $5.50 per watt installed. Add to the the solar renewable energy credits which you can sell each year. Those credits currently sell on the open market for $225-$250, and are set to double in two years. It makes the project very attractive. I have had the system for about two and a half years and figure my full payback will be reached in just under 4 years. Unfortunately for late adopters, the rebates are now 50% or less.

[chem geek]

I don't know what kind of solar panels you have, but take a look at this link that shows the change in efficiency at different flow rates. It doesn't drop nearly as fast as your power consumption will drop at lower flow rates. The tradeoff will be somewhat slower heating of the pool with much larger savings in electric cost. Very roughly speaking, in a variable speed pump, the GPM is roughly proportional to the RPM while the power consumption varies as the cube of the RPM. You can see from the solar panel efficiency graph that going from 4 GPM at 80% efficiency to 2 GPM at 70% efficiency isn't a huge drop in efficiency, but that the power consumption will drop by a factor of 8 (a little less than that due to "fixed" losses). With a 2-speed pump, you may not be at the "sweet spot" of reasonable efficiency and low power consumption, but you just might.

Richard