Share
Content Type:Q&A

Question:

Our facility has two indoor pools, containing a total of 199,741 gallons of water. The water is heated with a Coates compact electric boiler (270 kW, 480 volts, 325 amps, 3-phase, 60 cycle, operating psi = 30, operating temperature = 200°F). We keep the air temperature at 85°F, relative humidity at 40%, and the water is held at 83°F. Electricity costs 6 cents per kWh, with a demand charge of $2.75/kW. How can we determine what it costs to heat the pools?

Answer:

Actual operating costs for a swimming pool include:

  1. Pool evaporation losses
  2. Pool convection, conduction and radiation heat loss
  3. Lighting
  4. Domestic water heating
  5. Building HVAC

Since we do not have information on the pool lighting, building loads, HVAC or domestic water heating systems, we cannot calculate their energy use. We can, however, calculate heat loss from the pool, which can account for as much as 70% of the pool’s energy consumption.

I will assume the pool water and the air are already up to operating temperature. In order to maintain a swimming pool at a given temperature, we must supply sufficient heat energy to replace heat lost by convection, conduction and evaporation. Calculation of heat loss through conduction and convection is a straightforward calculation using the methods outlined in the ASHRAE Handbook of Fundamentals.

Heat loss due to evaporation is a little more difficult to calculate. The rate of evaporation depends on several factors – the air temperature and humidity, water temperature, air velocity at the surface, pool activity level. Pool activity is an important variable. Heat loss from an unoccupied pool can be one-fourth to three-fourths of the heat loss from a pool full of active people. In order to simplify the calculation, one of our engineers developed a spreadsheet to calculate energy loss for pools using the "Shah" method. The spreadsheet calculates swimming pool energy use and takes into account activity level, air and water properties, dehumidifier energy, etc.

The spread sheet is a powerful tool for optimizing swimming pool heating systems because it allows you to quickly see what happens to energy use when making changes to the pool operations. Using the spreadsheet, I calculated the annual cost of heating your pool assuming "no occupancy," "medium occupancy," and "high occupancy." I ran additional calculations to illustrate the effect that even a small change in water and air conditions has on pool energy use. Results are shown in the following table.

Case Air Temperature/ Humidity Water Temperature No Occupancy $/Year Medium Occupancy $/Year High Occupancy $/Year
0 85/40 83 $13,125 $17,545 $32,812
1 85/60 83 $7,778 $10,398 $19,446
2 85/60 80 $2,858 $3,820 $7,144
3 82/60 80 $6,746 $9,017 $16,864
4 81/60 80 $7,750 $10,360 $19,376

Case 0 is your pool as it is presently operating. The temperatures in this pool are a little higher than recommended for recreational pools and the humidity is set much too low. Low humidity levels increase the rate of evaporation and the cost of heating.

For the other cases we raised the relatively humidity to 60%. Evaporation losses for Case 1 are over 40% less than Case 0.

For Case 2 we lowered the water temperature to 80°F, keeping the air at 85° - annual energy use drops to about 20% of its initial value (Case 0).

Cases 3 and 4 show the effects of changing the air temperature. As the air temperature drops, the evaporation rate increases, as do the annual energy costs.

Topic: Building/Space Type--Sports/Recreation
Topic: Water Heating--General
Topic: Steam Systems--Boilers
Sector: Residential
Sector: Commercial
Content Type: Q&A
Keywords: energy cost analysis, swimming pools, evaporation losses, convection, conduction
ID:  4041