Energy Consumption

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Energy Consumption

  • Operating Conditions
  • Water Heating Energy Recovery
  • Space Heating
  • Space Cooling
  • Outdoor Air, Exhaust Air and Energy Recovery

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Energy consumption and performance implications based on the owner's choice of operating conditions and building envelope should be discussed in order to ensure there are no surprises. An all glass structure, for example, is going to be expensive to heat and difficult to keep condensation free in a northern climate.

A natatorium has 5 major areas of energy consumption:

Operating Conditions

Pool water heating and evaporation rates are always interrelated. Every pound of moisture evaporated to the space is a load to be dehumidified and it also represents heat lost by the pool water.  90% of a pool’s annual water heating cost is due to pool water evaporation losses. Every pound of moisture evaporated represents ~1000 Btu of heat lost from the pool water body and unless a pool is covered, they lose that heat (evaporate moisture) 24/7.

  • The warmer the pool water, the higher the evaporation rate.
  • The lower the space relative humidity level, the higher the evaporation rate.
  • The lower the room temperature (dew point), the higher the evaporation rate.

At the same water temperature a pool in a room at 78°F 50% RH will evaporate almost 35% more than that same pool in an 85°F 50% RH room.

While the space temperature should be dictated by the owner based on what satisfies their patrons, is it useful to be aware of a few guidelines that can help with energy consumption:

  • Maintaining the room air temperature 2°F - 4°F above the pool water temperature (86 degree maximum, per ASHRAE) will help reduce evaporation. This reduced evaporation in turn reduces the pool water heating requirement.
  • Do not introduce more outdoor ventilation air than required by code. In winter the space relative humidity levels will drop below 50% which increases the evaporation and pool water heating.

Water Heating Energy Recovery

When using a refrigeration based approach to control humidity, the dehumidification process captures energy in the refrigerant at the evaporator coil. The latent heat component is essentially the pool’s evaporation. Evaporation represents a significant portion of the pool's annual water heating requirements. If this energy is captured in the refrigerant, why not return it back to the pool water where it came from? This is free heat as a byproduct of dehumidifying the air. A Seresco dehumidifier with the pool water heating option has an enormous potential for energy savings.

A Seresco dehumidifier with the pool water heating option will capture and return the energy the pool water lost through evaporation. This process has an impressive COP of close to 8!

The use of the pool water heating option satisfies ASHRAE Energy Standard 90.1, otherwise a pool cover is required to meet the standard.

The Energy Cycle

The Seresco refrigeration based unit uses 100% of the compressor hot gas to heat the pool water and/or reheat the air. Returning this free energy back to the pool water or room air greatly reduces the annual heating costs. During the cooling season the dehumidifier is capable of providing 100% of the pool's water-heating requirement.

The mechanical refrigeration system approach to controlling the environment in a pool is a unique use of the refrigeration system. The evaporator controls the humidity while simultaneously the compressor hot gas can be used to heat the pool water and/or room air. Traditional air conditioning systems merely send the compressor hot gas outdoors to a condenser or cooling tower and do not tap into this heat source.

Adding the pool water heating option to your dehumidifier typically has a payback of less than one year.

Annual Savings from Pool Water Heating Option
Cooling Season Heat Source Average Pool Evaporation (lb/h)
20 30 40 50 100 150 200 300
4000 hours Gas $640 $960 $1280 $1600 $3200 $4800 $6400 $9600
Electricity $1880 $2820 $3760 $4700 $9400 $14100 $18800 $28200
2000 hours Gas $320 $480 $640 $800 $1600 $2400 $3200 $4800
Electricity $940 $1410 $1880 $2350 $4700 $7050 $9400 $14100
Table 6 - Annual Water Heating Savings from Pool Heating Option

Table 6 shows the annual contribution towards water heating from the dehumidifier while operating in cooling mode. A pool with a 50 lb/h evaporation rate and cooling season of 2000 hours would realize an annual savings of $2,350 if the primary source of pool water heating was an electric heater.

Calculations based on: 1000 Btu/lb latent heat of vaporization. Gas: $0.60 per 100,000 Btu, efficiency = 75%. Electricity: 8¢ per kWh

All systems require auxiliary pool water heaters. The Seresco unit will control their operation when it is not able to provide full water heating.

Space Heating

As with every other room being designed, the cooling and heating load calculations should be done for the natatorium. That is the only way to ensure the specific heating and cooling requirements are met. The room air temperature of an indoor pool facility is generally 10-15 °F warmer than a typical occupied space so the heating requirement per square foot of a natatorium will be considerably higher than a traditional room.

  • Outdoor air must be included in load calculations as it often represents up to 50% of the heating load.

Space Cooling

Most patrons prefer buildings with year round temperature control. Even though the space is generally 10-15 °F warmer than a typical room, most patrons would find it objectionable to be in a space that has no cooling at all.

Space cooling is a free byproduct from packaged dehumidifiers and chilled water systems. Providing year round temperature and humidity control can be provided by these systems. These systems dehumidify by cooling the air below its dew point and condensing moisture at the cooling coil. If the cooling load exceeds the standard output of a dehumidification unit, a larger unit with compressor staging is often specified.

Packaged Refrigeration or Chilled Water dehumidifiers provide summertime space comfort with no additional operating cost to the owner.

Outdoor Air, Exhaust Air and Energy Recovery

Outdoor ventilation air is essential for maintaining good IAQ in the pool and is a code requirement. The natatorium needs to be maintained at a slight negative air pressure, so warm “energy rich” air needs to be exhausted.

Outdoor air must be conditioned. It must be cooled and dehumidified in the summer and heated in the winter. In northern regions outdoor air has the biggest impact in winter where it reduces the space relative humidity levels and represents a significant portion of the natatorium’s heating requirement. In southern regions the outdoor air introduces a lot of moisture and increases the dehumidification load.

In northern regions outdoor air in winter may need to be heated 100°F just to get neutral to the space temperature in the pool!

The designer has several energy issues to consider:

  • Introducing more outdoor air than codes is not recommended:
    • In winter it will increase space heating and pool water heating costs significantly.
    • Too much outdoor air in winter can also lower the relative humidity levels to uncomfortable levels for the patrons. Yes, the pool room can actually be too dry!
    • In summer it can introduce so much additional moisture that larger equipment could be needed.
  • Warm “energy rich” air is required to be exhausted from the space to maintain negative pressure and good IAQ.

Energy recovery from the “energy rich” exhaust air to outdoor air should be considered.

Air-to-air heat exchangers are available for both sensible heat recovery and total energy recovery. Sensible only devices are used in Natatoriums. All sensible recovery devices are effective but some are better suited to provide a cost effective solution. Figure 12 shows two examples. There are several considerations to determining the need and feasibility of heat recovery:

  • Is the installation in a northern application?
  • Does the outdoor ventilation air need to be conditioned in order to avoid condensation when blended with the room air in summer and winter?
  • Are the outdoor ventilation air and exhaust air streams within close proximity to each other?
Figure 12 - Heat Exchanger
Figure 12 - Heat Exchanger

Heat recovery is generally packaged as part of a dehumidifier when outdoor or rooftop installations are specified. Figure 13 shows a schematic of the preferred heat recovery method used by Seresco. The heat recovery devices in Figure 12 require special and complicated air paths within the unit. This increases the unit size and cost and also has a large airside pressure drop which increase blower motor sizes and operating costs. Seresco has chosen a better solution and opted for a heat recovery option that can be packaged seamlessly within the standard dehumidifier or can be remotely installed in the ductwork.

Figure 13 - Packaged Heat Recovery
Figure 13 - Packaged Heat Recovery

Seresco provides unit mounted heat recovery using a glycol run-around loop. This approach to heat recovery offers the best performance and design flexibility while staying in the smallest possible cabinet. This heat recovery coil set fits directly onto the outdoor air and exhaust air openings already provided on the unit and does not increase the cabinet size. They are also easily sized to meet the specific requirements of your facility. The result is a compact, cost effective heat recovery option that actually fits into a mechanical room. The compact nature if this design results in lighter weight cabinets compared to units integrating plate heat recovery technology. This is an important consideration on applications where roof loads are a concern.

The glycol run around loop performance is engineered for a pool application. The coils are fully dipped for corrosion protection and there is no defrost or bypass mode on the coldest winter days. When you need the heat recovery the most, other heat recovery devices require that you bypass air to keep them from freezing up.

The introduction of the heat recovery coils into the existing air streams offers a substantially lower overall airside pressure drop compared to units with dual air paths and complicated internal air patterns. This configuration offers the end user the lowest possible operating cost while providing the best possible heat recovery effectiveness.

Adding the heat recovery option to your Seresco unit in a northern installation will typically realize a one year payback on your investment!

Seresco has developed a quick calculation to help determine the energy recovered and energy savings possible from the heat recovery coils.

Pool rooms are warm - It is surprising to note that heat recovery is viable even in a mild climate like Atlanta.

The savings are noteworthy even in a mild climate. The added cost of a heat recovery device generally pays itself back in less than two to three years in a mild climate.

Weather Data
City Average °F - T1 Winter °F - T2
Atlanta 61 17
Boston 51 6
Buffalo 43 2
Chicago 51 -8
Dallas 65 18
Denver 50 -5
Detroit 49 3
Minneapolis 45 -16
Indianapolis 52 -2
Nashville 60 9
New York 54 11
Oklahoma City 60 9
Pittsburgh 50 3
Portland, OR 53 17
Salt Lake City 52 3
Seattle 51 20
St-Louis 55 2
Toronto 46 -5
Table 7 - Weather Data
Heat Recovery Savings (Q) Analysis
Pool Location Atlanta
T1 Average Outdoor Temperature 61 °F
T2 Winter Design Temperature 17 °F
T3 Indoor Design Temperature 84 °F
V Outdoor Air Volume 3500 CFM
N Occupied Hours 12 hours
ER Electric Rate: 0.06 $/kW
GR Gas Rate: 0.65 $/CCF
GE Gas Heating System Efficiency: 60 %
Space Heating By: GAS
HE Heat Recovery Efficiency 50 %
Q {T3-T1} x 1.08 x V x {8760*N/24} x GE 190400 MBH
Annual savings from heat recovery device
$ {Q x $/CCF} / HE $2,062
Reduction in peak heating
Q1 {T3-T2} x 1.08 x V X HE 126,600 Btu/h
Table 8 - Energy Recovery Calculation