Tuesday, January 19, 2010

Indoor Pool Heating, comparisons with other activities

Hello, all few of you reading this:

A question arose today about how the energy use of a small indoor pool compares to other activities.
Interesting enough question that can probably be analyzed fairly quickly, assuming a spherical cow--> that is, back of the envelope.

The link to the spreadsheet is given here:

In Swimex pools, all heating and pumping is electrically driven, with heating performed by electric resistance.  I assumed an 8'x15'x5' pool with a thin fiberglass shell, exposed top, and resting in 5 inches of foam on the bottom.

Indoor air temperature assumed to be 74 deg F.
Pool Temp assumed to be 80 deg F.

Electricity numbers (kWh) are in Site Energy.
Source Energy numbers are arrived at by multiplying Site Energy by 3 and converting kWh's to BTUs (3412 BTU/kWh).

First I calculated annual heat loss through the fiberglass shell:  23.6 MBTU/year Source Energy
Heat loss via evaporation off the surface:  64 MBTU/year Source energy
Pumping energy used during operation: 8 MBTU/year Source energy

Total Source Energy used: ~100 MBTU/year

Alternative Calculation methods:
I found this spreadsheet linked to a website:

Plugging in the numbers into this spreadsheet, I get a number an order of magnitude bigger, with an annual heating cost of $23,000, which doesn't feel right at all.  So, I'll chuck that one for now.

Putting this into Perspective:
Alright, we've got 100 million BTU's per year.  Let's equate that to some actual energy services:
100 MBTU's equals:
767 gallons of gasoline per year.
18,000 miles in a 24 mpg car
15,000 lbs of CO2

That's pretty significant-- just for a small pool.  Part of the problem is that most of the heating energy is coming from an inefficient source-- electric resistance.  With a heat pump, we could increase the efficiency of the heating from 100% to 250-300% efficient (just a rough guess).

How Many kW of PV to make up for the energy:
Total Site Electricity is about 10,000 kWh per year for heating and pumping.
That would require about an 8.5 kW PV array given the Palo Alto conditions appropriate for many sites.

What if there was a cover on most of the year?
I didn't calculate this directly in my spreadsheet, but I can already tell that it would mostly eliminate evaporation from the pool and leave just conduction through the cover.
I will guess a 50% reduction in heating energy requirements-- let's call it 50 MBTU's per year of source energy instead of 100.

You can halve all the other impacts too-- we're still left with about 9000 miles of car travel a year, and 3.5 short tons of CO2 per year.

What about the structure?
Summarizing what I've assumed for the embodied carbon footprint of the structure.  The rule of thumb is that the embodied carbon footprint is about the same as 1 year of operational energy.  That's pretty generous, and usually applied to an insulated house.

About a ton of fiberglass and metal.  That's about 2000 lbs of CO2.  give or take-- remember, spherical cow.
About 120 cubic yards of excavation for the pit and mechanical room.  At  12 lbs per yard of excavation, we've got about 1400 lbs of CO2 there.  That includes excavation AND transport round trip to a place to dump the fill.
Fill in the concrete and steel:  about 15 cubic yards of concrete, at 0.3 lbs CO2 per lb of concrete is about  18,000 lbs of CO2.
Total is about 20,000 lbs of CO2 for the entire structure.

The carbon footprint of the structure and pool itself is about equal to 1000 gallons of gasoline, or 25,000 miles in an average passenger car.

That's it for today-- (that took a while!  whew!)


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