Active Solar
CAV Systems
Evaporative Cooling



CAV systems, active solar heating and evaporative cooling are similar in that they all can greatly affect the climate of a certain area even though all three systems perform this duty differently. Looking at the three systems compared to the main HVAC comfort parameters a well as other important design considerations, many difference and similarities can be viewed. 

All three systems have advantages and disadvantages, which sometimes balance each other out. The CAV unit works very well but is quite expensive.  The active solar unit is inexpensive to operate, but has high initial costs and only works under certain conditions i.e. sunny days. The evaporative cooler is also inexpensive, but only works well in hot and dry climates.

Looking at air velocity, there is a huge difference in how the three different systems can control it. The active solar heating canít control air velocity because it usually distributes heat by water in pipes.  The evaporative cooler can somewhat control the velocity in air if it has different fan speed settings.  The CAV unit can precisely control the air velocity since the airflow volume is kept constant in the zones. Looking at volume, the evaporative cooler and active solar heater cannot control the amount of volume of heat, cooling or air flow.  With the CAV system, volume can be controlled to any extent within the limits of the system.

The availability for a system to heat or cool is another important consideration in design.  The active solar can be used to cool an area but it is not very efficient.  The CAV and evaporative cooling are optimal for cooling air.  The CAV can be used in almost any environment and evaporative cooler works best in a hot and dry environment. The evaporative cooler is the only one of the three that cannot provide heat to an area.  The other two are designed to be able to handle heating loads. In terms of reliability, it is important for the HVAC equipment to be dependable. The active solar heater is not very reliable in conditions where solar energy cannot be access such as cloudy days.  The evaporative cooler and the CAV are fairly reliable however the evaporative cooler is not as reliable in humid climates.

Environmentally, there are also differences and similarities between the three systems. The active solar heating is very good for the environment because it uses energy from the sun, which costs virtually nothing and does not put any pollutants in the air. Similarly, the evaporative cooler also does not use pollutants and requires very little energy to run. On the contrary, the CAV system usually uses fossil fuels to run it which produce gases harmful to the environment.

Humidity, pressure, particulate control and ventilation all are important in HVAC comfort. The CAV system is the only one which is able to control all four of these components. Looking at humidity control, the CAV system is the best since it has the option of adding a humidifier in the system to add moisture to the air if it is too dry.  On the contrary, the evaporative cooler adds excessive moisture to the environment when the system is on and is difficult to control.  Since the active solar heater typically supplies its heat through heated water in pipes, it does not have control over how much moisture is produced. In terms of pressure, the active solar heater and the evaporative cooler do not control the pressure of the rooms or building that they supply.  Conversely, the CAV can precisely control the amount of pressure.  This is useful mainly in hospital and manufacturing where rooms need to be pressurized so when doors are opened the air flows from the high pressure areas to the lower pressure areas. In addition, the CAV has great control over the ventilation of the system. On the contrary, the evaporative cooler only vents through the open space that allows for heat exit and the active solar heater does not control ventilation. Lastly, considering particulate control, the active solar heater does not control particulates very well. However, both the CAV and evaporative cooler can trap particulates in the air.  However, the evaporative cooler cannot control how much it catches because it automatically catches most particles in the wetted pad.

The cost of a system can play a large role when choosing a HVAC system. The initial cost of active solar heater can be up to about $5000 whereas a CAV will cost less than that and an evaporative cooler is the cheapest, starting at about $100 plus the cost of installation. However, both the active solar heater and evaporative cooler both cost very little to operate and maintain compared to the CAV system.  The active solar runs of energy from the sun and the evaporative cooler can be plugged into an electrical outlet of 120 volts. On the contrary, one of the biggest disadvantages in the CAV system is its operating costs. These cost are very high do to the constant peak airflow in the system.  

In terms of area, the evaporative cooler covers a limited amount of area while the CAV and active heating can possibly cover a greater area. Depending on if the CAV system is a single zone or multizone dictates what areas the system is able to cover. Similarly, the active solar heater can cover a small area or a greater amount area, depending how big the system is. In regard to the amount of space needed for the different components of the system, the evaporative cooler uses the least amount of area. With active solar heating, the more space that the solar panels have, the more area they can heat. The CAV system can need a lot of area for the different components of its system, such as a boiler, cooling tower and ductwork. This depends on the loads, building types and other considerations.

After looking at all the about parameters and considerations, a comparison chart was created to clearly see the similarities and differences as well as the strengths and weakness of the different systems. The systems were ranked from 1 to 5, with 1 representing not existent or a very low value and 5 representing the highest value


Comparison Chart

  Active Solar CAV System Evaporative Cooling
Air Velocity 1 5 3
Cooling Availability 1 5 5
Efficiency 3 3 3
Environment Concerns 5 3 5
Equipment space 5 2 5
Heating Availability 5 5 1
Humidity Control 1 5 2
Initial Cost 1 3 5
Life span 4 4 3
Operating Cost 5 2 5
Particulate Control 1 4 4
Pressure Control 1 4 1
Reliability 1 4 5
Service Area 4 3 2
Ventilation 1 5 3
Volume 1 5 1

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This site was last updated 05/04/05