Times have changed regarding the design and program requirements for high schools. With these new program requirements and changes in the energy code, building systems and their integration into the building design play a larger role. There are many different space types within a high school. Each space has unique requirements which need different systems to maintain safety, comfort and functionality while performing within energy requirements. Each space does require four basic HVAC functions. How each of these are generated and operated will have a substantial impact on the energy performance of the building.
They are:
1) Heating., 2) Cooling, 3) Ventilation and 4) Temperature Control.
Heating
Hot water is now the standard for heating a building versus historical steam systems. There was a time when 180°F water would be the standard design to circulate throughout a building. This was done because existing boiler technology could not handle low return water temperatures or high temperature differentials. The major breakthrough in condensing boilers has led to significant increases in energy performance. Up to 97% efficient systems can be realized when condensing boilers are installed and operated correctly. Engineers are now successfully designing buildings with a hot water supply temperature of 120°F and return temperatures at or below 100°F.
Boilers mainly use natural gas. One way to reduce carbon footprint and further improve energy savings beyond condensing boilers is with a geothermal heat pump system. They can be distributed (small units serving each space) or a centralized heat pump which distributes hot and chilled water throughout the building. Geothermal heat pumps offer value reducing energy consumption in cooling mode, but its real value is reducing energy consumption in heating mode.
Cooling
Due to year round operation and the acknowledgement that thermal comfort leads to improved learning conditions, cooling has become the norm for today’s schools. Cooling is created through the refrigeration cycle. A compressor takes a low pressure gas and converts it into a high pressure/hot gas. This hot gas is then cooled into a liquid and the liquid passes through a metering device where it expands and absorbs energy… to create cooling. Whether the building has a chiller or direct expansion (DX) unit, it is the same principle. There are two major issues to deal with in improving efficiency:
1) Compressor capable of modulating to match building load
2) Lower the discharge pressure the compressor pushes against
The cooling load required for the building changes constantly due to environmental conditionals (solar load, infiltration, equipment usage) as well as space usage. If the cooling system does not have a compressor capable of modulating to match the load, the result is inefficiency and temperature control . A compressor controlled by a variable frequency drive can match the load of the building. It also has the benefit of working with a cooling tower which lowers the discharge pressure on the compressor. For these reasons, a water-cooled chiller is considered one of the most efficient solutions for cooling.
For further energy savings, consider a geothermal heat pump solution. Rather than utilizing the wet bulb from the cooling tower for heat rejection, the earth is used as the heat sink with its stable temperature lower than wet bulb.
Ventilation
Outside air requirements are provided via Code and are essential to a good learning environment. How many of us have been in congested conference rooms with no ventilation? After a short period of time the CO2 level substantially rises causing everyone to feel tired and lethargic. Unfortunately the side effect of more outside air is increased energy consumption. There are several ways to improve the energy performance while ensuring adequate CO2 levels:
1) Demand Control Ventilation – measure CO2 levels and adjust the quantity of air into space to meet minimum requirements during occupied hours
2) Energy Recovery – any outside air entering the building must also leave the building. By controlling how it exits the building and recovering the energy via enthalpy wheels, energy performance is improved.
Temperature Control
Each space is unique with special requirements to maintain thermal comfort. Building Management Systems (BMS) are now standard to monitor and control the operation of building systems. Each room (or temperature control zone) has a temperature sensor, which is constantly monitored. When the room is not within the required set point, the HVAC system will adjust until the room is back at set point.
There are a number of HVAC systems which can be installed for temperature control. Common options include Variable Air Volume Systems (VAV), Low Supply Displacement, Active Chilled Beams. It is important to understand that classroom require a high percentage of outside air so the installed system has to be capable of fulfilling that requirements.
One item of note, finned tube radiation has been a standard in the north east at windows. With enhancements to façade construction, window design, and construction quality, finned tube radiation has been successfully eliminated without any negative impact to the thermal comfort of the room for a VAV system with reheat.
These fundamentals of heating, ventilating, and air-conditioning (HVAC) for schools serve as the foundation to provoke discussion, the generation of suitable options for the project and the associated comparative analysis for the project team to make informed decisions. Lastly, each project is a unique opportunity… to make a difference.
Scott Robbins is associate principal at Vanderweil Engineers in Boston, Massachusetts.