Air-side System/Equipment

Displacement Ventilation

Technology outline:
Displacement ventilation uses a low-velocity stream of fresh cold air supplied near the floor to slowly "displace" the stale air up toward the ceiling from where it leaves the room. This stratifies the air in the room, with warm stale air concentrated above the occupied zone and cool fresher air in the occupied zone. The figure below illustrates the schematic of the system.

Schematic diagram of displacement ventilation. The text above describes the image.
Schematic diagram of displacement ventilation

How it can save energy:
The implementation of displacement ventilation reduces the energy consumption in several ways:

1.

The supply air temperature for displacement ventilation is usually 5°C - 8°C higher than that of conventional mixing ventilation. This allows higher refrigerant evaporator temperatures in the air-conditioning equipment, which reduces the temperature rise across the compressor and increases the coefficient of performance (COP) of the cycle.

   
2.

The stratified air in a space using displacement ventilation results in a higher average room air temperature than mixing ventilation resulting in reduced heat transfer through walls and roof of a building.

   
3.

For a system incorporating demand-controlled ventilation, the required fresh-air for a displacement ventilation system could be lower than that of mixing ventilation because light-weight pollutants (e.g. dusts) are trapped near the ceiling in the stratified air and can be removed easily through the ceiling exhaust air ducts.

How much energy can be saved?
The energy saving of this system depends very much on the control strategies of the air conditioning system, the building type  and the climate. As a whole, the electricity consumed for cooling is always lower for displacement ventilation than for conventional mixing ventilation.

To view project example, please click here.

Heat recovery equipment for air-side equipment

Technology outline & how it can save energy:
(a) Thermal wheel and Desiccant total energy heat recovery wheel
Thermal wheel is a heat transfer device with a rotating wheel, which allows the sensible heat transfer between the fresh air intake and exhaust air in an AC system. The air duct connections are arranged so that each of the airstreams flow axially through approximately one half of the wheel in a counter-flow pattern. The porous media that is heated from the warm duct airstream, rotates into the cold duct airstream where sensible heat is released.

Some thermal wheels are incorporated with desiccant materials such as silica gel which permits them to also transfer moisture from one air-stream to another. Thus, the moist air is dried while the drier air is humidified. In total heat transfer, both sensible and latent heat transfer occur simultaneously.

This flash illustrates heat recovery wheel. The paragraph above describes heat recovery wheel.

Heat recovery wheel

(b) Heat Pipes
A heat pipe heat exchanger is a passive energy recovery device. It consists of many tubes that divided into evaporator and condenser sections by a partition plate. Hot air passes through the evaporator side of the exchanger and the cold air passes through the condenser side in a counter-flow arrangement to allow heat transfer between the two air streams. The following is a schematic of a typical heat pipe.

A typical heat pipe. The text above describes the image.
A typical heat pipe

In order to enhance dehumidification application, heat pipe coils are installed in conjunction with dehumidification coils. In this case, a "wrap around" configuration of heat pipe coils is adopted. The heat pipe wraps around the cooling coil with one section of the heat pipe coil upstream and one section downstream as shown in the following diagram.

Application of heat pipe in HVAC. The text above describes the image.
Application of heat pipe in HVAC

As shown in the above figure, heat pipe contributes to the HVAC by pre-cooling and reheating the air. The pre-cool section of the heat pipe is located in the incoming air stream. When warm air passes over the heat pipes, the refrigerant vaporizes, carrying heat to the second section of heat pipes, placed down stream. Air passing through the evaporator coil is assisted to a lower temperature, resulting in greater condensate removal. The "overcooled" air is then reheated to a comfortable temperature by the reheat heat pipe section, using the heat transferred from the pre-cool heat pipe. This entire process of pre-cool and reheat is accomplished with no additional energy use.

Run-around coil system. The text above describes the image.
Run-around coil system

(c) Run-around coils
The run-around coils are finned-tube copper coils placed in supply and exhaust airstreams. The coils of the run-around system are via piping, and a pump circulates water, glycol or thermal fluid solution. The system utilities the energy from the exhaust air stream to pre-condition the outdoor air. In summer, the exhaust air from the air-conditioned space cools the circulating fluid in the coil. The cooled fluid is then pre-cool the outdoor air. In winter, the process is reversed; heat is extracted from the exhaust air and then transferred to the make-up air.

 

Liquid desiccant air conditioning system

Technology outline:
A liquid desiccant air conditioner removes moisture and latent heat from process air via a liquid desiccant solution, usually lithium chloride. In a typical liquid desiccant system, the desiccant is distributed in a chamber (conditioner) using spray nozzles. In the chamber, the desiccant contacts with the process air steam to absorb the moisture from the air. As the moisture is absorbed into the desiccant liquid, the diluted desiccant solution is then pumped to another chamber (regenerator) where heat is supplied to remove the water from the desiccant into an exhaust air stream. The dry air is then pumped to the conditioning chamber to be reused.

Schematic diagram of a Liquid desiccant air conditioning system. The text above describes the image.
Schematic diagram of a Liquid desiccant air conditioning system

How it can save energy:
The liquid desiccant air conditioning system preconditions outdoor air to reduce its moisture content before they enter the building. In this way, the latent load of the outdoor air is handled by the system. At typical design conditions, no sensible cooling is provided to the building by the system. However, at lower outdoor wet bulb temperature, the air delivery temperature is lower and some sensible cooling is provided in addition to the latent cooling capacity. As a result of this make-up air system handling the entire humidity load of the building, the remaining air conditioning load would be all sensible, allowing the air conditioner to be operated at increased evaporating temperature, improving the coefficient of performance (COP) relative to a conventional chiller.


The regeneration process requires heat input, usually low-grade energy such as solar energy and waste heat of industrial processes are used so that it consumes little primary power.

How much energy can be saved?
The energy saving of the liquid desiccant air conditioning system depends on the outdoor air condition and system configuration.


A study in the U.S shows that, in compare with a conventional primary air handling unit, the liquid desiccant air conditioning system can offer 20-25% energy saving for outdoor preconditioning process [1].

The energy saving of the liquid desiccant air conditioning system depends on the outdoor air condition and system configuration.

Further sources of information:
This web page has hyperlinks which may transfer you to third-party website.National Renewable Energy Laboratory

Microchannel heat exchangers

Technology outline:
Microchannel heat exchangers transfer heat through a number of flat fluid-filled tubes containing small channels while air travels perpendicular to the fluid flow. Fan-fold fins with louvers connect the adjacent tubes. In this way, air passing over the heat exchanger has a longer time compared to current fin-tube heat exchangers.

How it can save energy:
Microchannel heat exchanger provides improved heat transfer as compared to conventional heat exchangers due to:

  1. The small refrigerant flow passages result in high refrigerant side heat transfer
  2. Flat orientation of the tubes reduces airside flow resistance, leading to either increased air flow or reduced fan power either of which can improve overall system efficiency.

The increase in heat exchanger effectiveness allows the microchannel heat exchanger to be smaller and yet have the same performance as a regular heat exchanger. The smaller size of the exchanger reduces the refrigerant pressure drop, improving overall compressor performance.

How much energy can be saved?
Energy savings greatly depend on the size of the heat exchanger, the application, and how other refrigerant components are optimized. In general, these heat exchangers are approximately 15% more effective than conventional fin and tube heat exchangers.

To view project example, please click here.

Radiant ceiling cooling

Technology outline:
Radiant ceiling cooling system, also know as "chilled beam" system, incorporates pipes in the ceilings of the buildings through which cold water flows. The pipes lie close to the ceiling surfaces or in panels and cool the room via natural convection and radiative heat transfer (as shown in the figure below). However, condensation caused moisture to accumulate on the cooled surfaces. This damages the ceiling materials. Therefore, a dedicated outdoor air system (DOAS) may be required to manage to outdoor air humidity.

Principles of radiant ceiling panel cooling. The text above describes the image.
Principles of radiant ceiling panel cooling

How it can save energy:
One of the basic energy savings mechanisms is the ability to operate with higher chilled water temperature, allowing the chiller evaporator temperature to be correspondingly higher, and thus reducing the chiller energy consumption. The chilled water temperature required in this system is generally 3-5°C higher than the conventional chiller water systems.


Other than higher chilled water temperature, the system can save energy due to reduced air flow. When implemented with the primary air handling unit, the radiant ceiling cooling system can save energy by reducing total ventilation air flow and by handling sensible cooling loads more efficiently. This leads to a reduced power for ventilation (only 25% to 30% of the air flow rate required for peak cooling loads in an all-air system).


How much energy can be saved?
According to a study in the U.S., when combining with a primary air handling unit, the radiant cooling system can reduce cooling ventilation energy consumption by 15%-20% relative to a VAV system. [1]

 

Reference

1. Roth, K. W., et al, 2002, Energy consumption characteristics of commercial building HVAC systems volume III: Energy saving potential, Building Technologies Program, Department of Energy, United States

   
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