Technology Overview: Rooftop Unit TES Systems

October 16, 2013
 
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Thermal energy storage (TES) systems can also be used with commercial and residential packaged rooftop units (RTUs), to shift energy use to off-peak periods.  RTU-TES systems can be designed for new construction or retrofit applications.

Image: Smart Planet (Ice Energy)

TES Integrated Packaged Rooftop System

Rooftop unit TES technology works with any refrigerant-based HVAC unit, with some requiring no changes inside existing RTU equipment and others requiring the installation of an additional indoor cooling coil. RTU-TES systems can shift 90-95% of energy used for HVAC from on-peak to off-peak hours. RTU-TES systems are typically used for commercial facilities of 50,000 square feet or less, as well as for residential single and multifamily buildings.

Operation of RTU-TES Systems

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RTU ice storage is one type of Thermal Energy Storage (TES) technology. TES is the most popular way to implement the demand-side energy management strategy known as 'Permanent Load Shifting' (PLS).

An RTU-TES system is essentially an ice storage tank with a self-contained ice making system. During “charging” mode (typically at night), the RTU-TES unit is used to freeze up to 450 gallons of water, depending on required load. The ice is stored until cooling energy is needed the following day.

On the following day as outdoor temperatures increase and cooling capacity is required during ‘on-peak’ periods, the system switches to the ice cooling (“discharging”) mode. In this mode, the RTU’s compressor and condensing units shut down and the stored ice is used for cooling. If the cooling coil is shared, refrigerant is routed from the RTU through the RTU-TES unit. Ice is slowly melted as it cools the heated refrigerant returning from the cooling coil, and ice-cooled refrigerant is then pumped back to the evaporator coil of the RTU, providing cooling to the building. If there is a separate cooling coil for the ice system, cooling is provided using the ice coil only.

After all the ice is melted (usually after on-peak hours) then cooling load is transferred back to the RTU and charging may also begin again.

RTU-TES Components

  • Insulated storage tanks
  • Controllers to manage refrigerant flows, energy consumption and mode controls
  • Copper coils and refrigerant pumps
  • Compressor and condensing unit

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