Hi HVAC Efficiency: 3 Part Series: Part 1: Innovations Offer Reductions In HVAC Energy Consumption But Are Often Ignored.

Hi HVAC Efficiency: 3 Part Series: Part 1: Innovations Offer Reductions In HVAC Energy Consumption But Are Often Ignored.

In recent years, the industry has devoted a great deal of effort to reducing the amount of energy used to operate buildings. During that time, a variety of innovative HVAC technologies and design strategies have been developed and proved in a wide range of projects. Those approaches, however, are often ignored when HVAC options are being weighed. That's unfortunate: Wider use of those HVAC strategies represents a substantial opportunity to cut energy consumption, since more than one third of the energy used in a building is for heating, cooling, and ventilation.

Those strategies are also crucial for projects aiming to achieve net-zero energy use. A net-zero energy (NZE) building is one where the total energy consumed over a period of one year, minus renewable energy generated on-site, is equal to or less than zero.

Designing a very efficient HVAC system, whether it's for a net-zero building or not, can be a significant task, and it is best accomplished through an integrated, holistic design approach. This approach requires the commitment and contribution of owners, architects, engineers, contractors, and other specialists. Building owners and facility managers play an integral part even beyond the design stage: Even the best-designed projects rely on the user's motivation to operate the building with a high level of energy efficiency — especially when striving for net-zero energy.

The appropriate selection of HVAC systems offers many opportunities to achieve great efficiency. But the first step in designing an efficient HVAC system, regardless of whether the project's goal is to achieve net-zero energy use, is to cut the demand for energy. This is accomplished by reducing building internal loads, by improving building envelope performance to reduce solar heat gains and conductive losses, and, at the same time, by maximizing the use of daylight.

Depending on the climate, cooling demand can be reduced by increasing building insulation and installing high-performance glass. Light-colored, reflective surfaces on roofs and walls and radiant barriers within ceilings are also important to avert solar radiation.

In recent years, there have been developments in dynamic building envelope technology that can alter performance in order to withstand peak periods. Smart glass or self-tinting products are controlled based on an electric signal. Phase-change materials can be installed in drywall in order to maintain surface temperatures during extended hot periods.

Internal electrical loads for lighting and power systems should also be reduced as much as possible to increase efficiency. About 39 percent of building energy use goes to lighting and office equipment. Reducing internal electrical loads also cuts the demand for cooling: For each kW consumed, a percentage of waste heat must be cooled.

PART 2: Consider These Passive Or 

Energy-Efficient Active HVAC Systems:

Once heat gain is reduced, passive HVAC systems or more energy-efficient active systems can be installed to satisfy occupant comfort.

Passive systems are effective in net-zero buildings because they allow the user to maintain comfort in a space without the need for energy. Common passive techniques include natural ventilation and the use of thermal mass.

Natural ventilation relies on outdoor air and appropriate high/low window placement to ventilate, bring in fresh air, and relieve hot air. Exposed thermal mass can often be provided in the form of concrete, water, or other materials to evenly regulate surface temperatures, even when exposed to heat. Exposed thermal mass is often used in conjunction with natural ventilation in order to pre-cool surfaces at night and store this coolth for daytime use.

Passive systems have two limitations: They can be slow to respond to changes in load, and they can have limited capacity. This makes the load-reduction strategies even more important. In mixed-mode buildings, passive systems are supplemented by active HVAC systems to operate during periods when outdoor temperatures are high or low.

Energy-Efficient Active Systems:

When it comes to active-system design strategies, it's important to remember that any measures that can help occupants maintain comfort without turning on building HVAC systems should be considered. Local personal workstation cooling systems or high-volume, low-velocity ceiling fans may allow facility staff to increase building temperature set-points while maintaining comfort. The premise of "adaptive comfort" is that people can accept a wider range of temperature if they have the means to make adaptive, local adjustments such as altering air velocity, modifying clothing levels, or changing the activity schedule.

For heating applications on small- to mid-sized commercial net-zero projects, electric heat pumps are often the preferred means for generating heat. The reason is that they can take advantage of electricity from on site renewable sources (e.g., from photovoltaics or wind turbines); that's why net-zero projects use natural gas heating systems less often than other types of projects. Other sources of heat, such as solar thermal collectors, are common ways to offset the electrical heating requirements.

Radiant systems, such as chilled or heated floors and ceilings, are effective for net-zero applications both in heating and cooling. These systems temper the environment without needing a fan to circulate air. Air-plus-water systems, such as active chilled beams, can also offset fan energy. A pump is required to circulate water; however, water is a much more efficient medium for transferring heat or cold than air.

For applications requiring large quantities of air, either due to ventilation or cooling demand, a means to offset fan energy should be considered.

The conventional method for doing this is to reduce supply air volume during temperate periods using a variable air volume (VAV) system. A VAV system uses a variable frequency drive in conjunction with the fan to modulate speed. Because fan power depends on airflow and static pressure, some systems look to save energy by reducing static pressure. An underfloor air distribution (UFAD) system can accomplish this goal by moving low-velocity air through a raised floor cavity to reduce the length of high-velocity air ducts. A UFAD system also supplies air at a higher temperature than VAV systems and often uses outside air to provide free cooling, leading to greater efficiency.

For larger applications, central plants employ water-cooled chillers with heat rejection from cooling towers. For this size of net-zero project, efficiency would be the driver for all mechanical components. There are opportunities, however, to use natural energy sources such as geothermal heating/cooling to modulate temperature in the HVAC fluids.

PART 3: Net-Zero Energy Buildings: Waste Heat Recovery And Renewable Energy:

Waste heat recovery and renewable energy are important strategies in net-zero energy buildings.

Because natural gas systems are less often used in net-zero buildings, there are fewer opportunities to recover energy from the heat-intensive processes. If natural gas is not used for heating, a project is unlikely to use fuel cells or micro-turbines, eliminating the ability to recover waste heat from those energy-generating systems.

On many projects, waste heat sources occur within the exhaust air streams or in condenser exhaust from heat pumps or chillers. Depending on the use of the incoming air and the quality of the outgoing air, heat can be transferred using air-to-air heat exchangers, air-handling-unit run-around coils, and thermal energy wheels. These systems either directly or indirectly transfer heat from one air stream to the next.

Another source of waste heat is the exhaust air from heat pumps or condenser water from chillers. If there is a simultaneous demand for heating (possibly in domestic hot water), this waste energy can be reused as a heat source.

Renewable Energy and Building Automation:

The steps outlined are important in another way for projects aiming to achieve net-zero energy use. Those projects decide which renewable power generation system (photovoltaic cells, building-integrated wind turbines, bio-fuel, or some other means) best matches the building demand and profile for the project type and region. Measures that reduce demand also help to reduce first cost of the generation system, and make it easier to deal with the source's intermittent power production. The protocol we've outlined makes it easier to size energy-generating systems at an appropriate and affordable capacity.

Building occupants also play an integral role in efforts to achieve energy efficiency, especially in net-zero buildings. If the user is unaware of the energy consumption in relation to the building energy generation, there is little chance of achieving a neutral energy balance (i.e., net-zero energy use) at the end of the measurement period. Building dashboards allow the user to monitor building energy consumption and to better understand when manual systems should be operated. For instance, the building dashboard may notify the user when outdoor air temperatures are acceptable for opening a window or inform the user when energy conservation may be needed on a cloudy day.

The design of a highly efficient HVAC system, especially for a net-zero project, is a complicated puzzle that requires all members of a project team to engage in the process. More than ever, there is reliance on all members to cooperate, motivate, and inspire their counterparts to achieve the common goal of energy neutrality. The reward is a net-zero building that can meet the needs of the present while having "zero" impact on the ability of future generations to meet their own needs.

Bruce McKinlay, PE, LEED AP, is a principal with Arup in Los Angeles and leads the firm's commercial property market in the Americas. He has collaborated on numerous sustainable projects where an integrated design approach has led to greater energy efficiency. He can be reached by email by clicking the following link here.

Jamey Lyzun, PE, LEED AP, is an associate at Arup and has 14 years of experience on projects. He has applied many of the techniques outlined in this article to assure that integrated strategies at the design phase can lead to low energy and sustainable outcomes during operation. He can be reached by email by clicking the following link here.

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