Airside and Waterside Economizing
With rising energy prices across the country, it is critical for building owners to pay close attention to how much energy their buildings are using on a daily basis. The cost of energy for a typical building may be one of their biggest expenses, so finding any possible way to cut those costs is of extreme importance. There is a long list of measures that can be employed to reduce the energy consumption of a facility, many of which have already been discussed in separate articles or will be added in the future. The purpose of this article is to discuss the mechanisms, benefits, and distinctions of airside economizing and waterside economizing.
Airside Economizing
Airside economizing is an energy savings measure that aims to reduce the energy necessary to cool a facility. Many energy conservation measures, or ECMs, seek to accomplish the same goal, so what makes this measure unique? Let’s start with the requirements. Airside economizing requires four types of airstreams: exhaust, return, outdoor, mixed, and supply. If a ventilation system is make-up air only, meaning 100% outdoor air with no return air mixing, airside economizing would not be applicable.
Now we need to understand the circumstances of when this energy saving measure will be effective. First, let’s think about a typical ventilation system. Most ventilation systems are designed to bring in the minimum necessary amount of outdoor air. This because, most of the time, over ventilation will result in more energy expenditure. For example, if it is 30 °F outside and the building is calling for heating, heating a higher volume of 30 °F will require more energy from the heating system. And vice versa for the cooling system. If it is 95 °F outside and the building is calling for cooling, cooling a higher volume of 95 °F air will require more energy from the cooling system.
Airside economizing enters the picture when bringing in more outdoor air actually reduces energy. This occurs when all of the following happen:
The building is calling for cooling
Outdoor air temperature is lower than the return air temperature
Outdoor air humidity is not overly high
When might all of the above circumstances happen? It will happen when there is a high internal heat load (lights, appliances, too many people, etc.) or excess solar heat gain causing internal temperatures to rise despite outdoor air temperatures being cold.
If all of the above do occur, the outdoor air damper should increase to the optimal position. What is the optimal position? The optimal position is the position in which the mixed air temperature is lowered to point right above needing to be reheated. As efficient as it may be to cool a building using freezing cold outdoor air, we cannot allow air that’s too cold to leave the supply vents and hit occupants. Let’s look at an example:
Outdoor air temperature (OAT) = 45 °F
Return air temperature (RAT) = 75 °F
Outdoor damper position (OAD) = 25%
Cooling supply air temperature setpoint = 55 °F
Supply airflow = 10,000 CFM
Mixed air temperature = OAT * OAD + RAT * (1 – OAD) = 45 °F* 25% + 75 °F * (1 – 25%) = 67.5 °F
Cooling energy = CFM * 60 * ρ * cp * ΔT = 10,000 ft³/min * 60 min/h * 0.08 lb/ft³ * 0.24 BTU/(lb °F) * (67.5 F° – 55 °F) = 144,000 BTU/h
Now let’s implement airside economizing, with the damper going to the optimal position:
Outdoor air temperature (OAT) = 45 °F
Return air temperature (RAT) = 75 °F
Outdoor damper position (OAD) = 67%
Cooling supply air temperature setpoint = 55 °F
Supply airflow = 10,000 CFM
Mixed air temperature = 45 °F* 67% + 75 °F * (1 – 33%) = 55 °F
Cooling energy = 10,000 ft³/min * 60 min/h * 0.08 lb/ft³ * 0.24 BTU/(lb °F) * (55 F° – 55 °F) = 0 BTU/h
And what would happen if we went beyond the optimal position:
Outdoor air temperature (OAT) = 45 °F
Return air temperature (RAT) = 75 °F
Outdoor damper position (OAD) = 100%
Cooling supply air temperature setpoint = 55 °F
Supply airflow = 10,000 CFM
Mixed air temperature = 45 °F* 100% + 75 °F * (1 – 100%) = 45 °F
Cooling energy Reheat energy = 10,000 ft³/min * 60 min/h * 0.08 lb/ft³ * 0.24 BTU/(lb °F) * (45 F° – 55 °F) = - 115,200 BTU/h
Waterside Economizing
Waterside economizing operates in a similar manner to airside economizing. The main difference is using a water loop to provide “free cooling” as opposed to outdoor air. All of the same requirements for airside economizing to be effective are relevant to waterside economizing as well. So how does this energy savings measure operate? For this to work, we will need at least the following:
Cooling tower
Heat exchanger
Let’s look at a simple example. An office building has a cooling tower, water cooled chiller, and plate and frame heat exchanger. The cooling tower feeds both the condenser side of the chiller and the heat exchanger. There is a diverting valve that decides if the cooling tower water enters the chiller or the heat exchanger. The heat exchanger and chiller share supply and return chilled water piping to the building air handlers, with some additional diverting valves to make sure there is no backflow.
The temperature of the cooling tower water depends on outdoor air conditions, specifically outdoor wet bulb temperature (see article on Psychrometrics). A cooling tower will be able to lower condenser water down to the wet bulb temperature plus an approach (normally around 7 °F). The chiller has a leaving temperature setpoint of 50 °F. If the cooling tower can generate water equal to or lower than 50 °F, the water should bypass the chiller and use the heat exchanger loop. That is assuming that the building is calling for cooling. If the cooling tower cannot generate water equal to or lower than 50 °F, it should follow a condenser water reset sequence. Through bypassing the chiller, the facility is able to be completely cooled without the use of the chiller.
See below diagrams for standard operation and waterside economizing operation:
Standard Operation
Waterside Economizing Operation
