Monday, March 29, 2010

Pumping pool water

I've been looking into the costs of pumping water through the filters and valves and pipe work around the pool, and it's quite interesting. For context, you need to understand that we live in California, in the San Francisco Bay Area (known to our utility PG&E as area X). That means that we're going to be using the pool 9 months a year, and the marginal electricity we'll be using for the pool will cost us $0.424 to $0.497 per kilowatt-hour (Those are the charges for >24.2 kWh/day and >36.3 kWh/day). That sounds like a lot of money, and it's going to sound worse as you read below.

In 1997, marginal electricity with a basic residential rate was $0.122/kWh, so the average cost growth since then is 10.0% per year. That's substantially faster than the discount rate of 6% per year that I usually apply to future money. If the cost growth of energy exceeds the discount rate into the future, it means that the energy I spend to pump pool water 10 years from now will cost more in current dollars than the energy I spend today.

You can't extrapolate a growth curve indefinitely into the future, and the price of electricity in current dollars cannot rise without limit. That said, I'm fairly confident that over the next 40 years the price of electricity will at least keep up with a 6% discount rate. That means that I'm predicting that the electricity the pumps burn 40 years from now costs me just as much, today, as the electricity that the pumps burn today.

I want to compare the costs of running the pool with the cost of building it, so I can make decisions about what sort of equipment to install. In the face of electricity that does not discount into the future, I have to put some sort of time limit on how long the pool will be operated. I'm going to pick 40 years. At that point my grandkids are likely to have learned to swim, and there is an about even chance that I'll be dead and won't have to worry about money any more. Over 40 years (9 months a year), each kilowatt-hour burned, per day, costs a total of $4643 in 2010 dollars. The bottom line is that, had I built my new pool in a standard fashion, the electricity to run it would have cost over $150,000 in present value. That's more than the pool! Instead, I've spent a few thousand dollars improving the efficiency of the plumbing so that my projected present value cost is more like $40,000 (which can be improved further).

Those of you with astronomical electricity bills, and pools, may be wondering how did I do that?

Let's get started with a simple result that is relatively new for the residential pool industry: slower is better. If I run a pump at 42 GPM for 16 hours, it'll turn over about 40,000 gallons. If, instead, I run the pump at 84 GPM for 8 hours, it'll move the same number of gallons. However, and this is really important, the pump has to push about twice as hard to move water twice as fast. Power is pressure times flow, so pumping twice as much water up twice the pressure head takes 4 times as much flow power. 4x the power in half the time is 2x the energy, and energy is what you pay for. Bottom line: slow the pump down.

You should also get the most efficient pump possible. If you don't have solar and so don't need variable head, and can mount the pump below the level of the water surface, then get something like a Sequence 5100. Otherwise get an Intelliflow 4x160, which is what I got.

Slower-is-better isn't the complete answer, because of variable pump efficiency and in-line spring-loaded valves. So most pools won't run their pumps 24 hours a day, as I do.

Note that I talked about flow power in the paragraph above. Flow power is pressure (e.g. psi) times gallons per minute. Try it. Most pumps turn electricity into flow power with an efficiency that varies between 15 and 50%. 15% is the efficiency at low speeds (right before the pump stalls and efficiency goes to zero), and 50% is the efficiency at something near top speed. Variable-speed pumps, like the Intelliflow 4x160, usually hit their maximum efficiency at high RPM settings and high flow rates (but I include a counterexample in my spreadsheet link below). As a result, when you run the pump more slowly, less energy is required to move the water, but the pump takes more electricity to produce each joule of flow energy. You can see the pump power and head curves for the 4x160 on page 47 of the user's manual. And I've extracted the numbers off the chart and done efficiency calculations here. Bottom line: best efficiency is at 30 GPM for a normal pool. I'll be forced up to 42 GPM (at a measured 31% efficiency) because the pool is big.

The analysis gets a little complicated because most pools have in-line spring loaded valves, which should be banned. Many solar installations suggest a check valve between the filter and the water line going up to the roof, because the panels drain at the end of the day and you don't want those panels draining backward, through your filter, taking all the scum in there back out through your skimmer. An innocuous little check valve seems like just the ticket. One wrinkle is that the check valve requires something like 1 psi to open, and since the flow is working against that spring, there is a 1 psi drop across the valve at all flow rates. Another wrinkle: yet another spring-loaded valve, in the heater this time, which drops about 5 psi. I'll get to that later.

1 psi doesn't seem like much until you consider that 40,000 gallons pushed through 1 psi at 31% efficiency is 0.935 kilowatt hours. Still not much? The present value is $4343. It's actually a bit worse than that because the constant pressure drop causes the pump to stall at very low speeds, which changes the best operation point to something faster and thus even less efficient.

My last pool (not my design) ran the pump at about 35 psi. As I said, the present value of each psi is $4343, so if this pump had to do the same thing, the present value would be $150,000. That number is so large that you are thinking that it is funny money. It's not. That is this year's electric bill for $3800, and next year's bill for $4000, and the year after's bill for $4200, and so on. It's committed money: if I turn off the pump, the pool turns into a pond and the City folks come by and suggest how to abate mosquitoes. How did I end up with this problem?

Pool equipment is generally designed for low equipment cost, not low energy cost, because the equipment cost is what most homeowners look at. But when we look at cost of energy to drive this equipment, it becomes clear that most pool equipment design is completely insane. The details will come in following blog posts, but in short:

PartCostPressure drop @ 42 GPMPower costAlternativeCostPressure drop @ 42 GPMPower cost
Rooftop pressure relief to drain solar panels$508.6 psi for 2-story house$37,400Extra 3-port valve and parallel drain to pool$4000.1 psi$430
Gas heater with internal spring-loaded valves (check page 9)$18005 psi$21,700Bypass heater with 3" 3-port valve and an actuator$20500.1 psi$400
DE filter with multiport valve (check page 8)$7004 psi$17,400DE filter with 2 3-port valves and ozonator$21001 psi$4,300
100 feet of 2" pipe$601.19 psi$5,170100 feet of 3" pipe$1100.173 psi$751
2" check valve$451 psi$4,3003" 2-port valve with actuator$2500.1 psi$430
2" 3-port valve$450.5 psi$2,2003" 3-port valve$900.1 psi$430

I've implemented all but one of these on my pool, and it works: I can pump 14 gallons/minute (nearly twice what is required) up through solar panels on my two-story house with 9.5 psi from the pump, and I can pump 40 gallons a minute through the filter at 6 psi. With a fix to bypass the heater (I didn't see that one ahead of time), the pressure drop from circulating water through the solar panels should come down to 5 psi or so.

If you work at Hayward or Pentair and you are wondering how you might fix up your product line to make it more attractive, let me make the following suggestions:
  • A $1500 variable-frequency pump with a proper volute, achieving 80% efficiency when operating at 30 gallons/minute and a 5 psi would be a game changer, as it would cut most pool owners' electric bills in half. It would require 3" ports. Please make the main shaft seal ozone compatible.
  • Rewrite your manuals to show pool installers how to use 3-port valves to bypass the heater and drain solar panels without spring-loaded pressure relief or check valves. I'll have diagrams for this on my blog shortly.
  • A DE filter to go with that snazzy pump, with 3" ports and a valve system with less than 1 psi total drop when clean at 40 gpm. Make the grids ozone-compatible (stainless steel?), and figure out a way to either send the separated air back to the ozone generator or through a catalyst to break down the ozone and nitrogen oxides.
  • Introduce or work with Del to produce an ozone generator which has the ozone passively sucked into the intake of the main pump. Alternatively, figure out how to make a low power air pump that forces the ozone into the high pressure water stream after the filter. The current Del air pump is far too power hungry.
  • 3" sweep 90 degree and 45 degree elbows, with interiors matched to PVC schedule 40 IDs. There is a trick where you twist the flow as it goes through the corner which might improve losses a bit.
  • 3" valves with smooth interior bores. This could be the breakthrough that gets people to take you as seriously as Jandy. The gussets on the valve door interiors now save tens of cents of plastic and cost homeowners hundreds of dollars.
  • Since all the big pipework and valves will take up a lot of space, pay careful attention to how all the bits fit together into the space allowed for existing pool equipment pads. Maybe a replacement for the multi-port valve which is made of 3-inch Jandy-type valves would work.
  • Fix the pool control systems so that a basic control box:
    • can handle 10 valves,
    • connects to (and comes with) a flow meter that works down to 10 gpm,
    • and has thermistor inputs for solar return as well as intake water, so you can calculate how many BTUs are coming off the roof.
I think that's enough for now. I've been tweaking this post for 10 months, it's time to post!