If a building owner has enough money for renewable-energy systems and the space to install them, any designer can achieve net-zero performance. Achieving net-zero performance economically, however, is a different matter.
Although the concept of "net-zero energy" has been with us since the energy crisis of the 1970s, the HVAC design industry still relies almost exclusively on fundamentally flawed, overly complex, mass-produced solutions that cannot be made to function properly. To achieve net-zero designs, we need to get back to engineering basics and practice fundamentally efficient design. We need custom solutions, and we need the skills to correctly evaluate and holistically apply strategies to meet a facility’s particular needs.
It is not a lack of technologies or barriers imposed by the laws of thermodynamics that impede our progress. It is our unwillingness to expand our horizons, analyze opportunities, develop application-specific solutions, and adopt appropriate techniques.
The real barriers to technological progress are those we impose on ourselves; it is our failure to use our skills and imagination that limit our ability to understand and properly analyze the opportunities and technological misconceptions that inhibit us from considering the technologies most necessary to practical solutions.
At this point, I think some important observations need to be shared:
- Every element of common HVAC systems is designed to consume, not conserve, energy. This severely limits systemic efficiency.
- The air-side economizer does not save energy, it discards potentially recyclable assets. In the process, it effectively compromises ventilation.
- Ventilation is energy-intensive because most common HVAC systems have no mechanisms to either efficiently process or effectively manage it. They simply fail to address the challenge.
- Reheat is pure waste. It actively defeats other energy-conservation measures and increases cooling energy use between one and five units for every unit of reheat energy used. There are no applications that justify its use.
Conventional HVAC systems throw away as much as 85 to 95 percent of the energy expended in buildings. However, with innovation and careful engineering, most of that waste can be eliminated or recycled. This was demonstrated at Wausau West High School where air-quality problems in an existing and uninsulated building were eliminated using 100-percent outdoor air, and total energy usage was cut 70 percent. This in spite of the fact the facility had previously been through a major energy-conservation project (see "Regenerative Dual Duct: A Case Study," HPAC Engineering, January 2009, or visit http://bit.ly/dualduct).
Similar results can be realized routinely. But this means building HVAC systems around different technologies to achieve different design objectives. It means applying a different system-design paradigm through which energy is recovered and/or transformed and new energy expenditures are avoided. The fact it has been accomplished defeats all arguments to the contrary.
Where We Need to Improve
The lack of an unambiguous, absolute, and universally accepted definition of a "high-performance" HVAC system is a huge problem. Similarly, when it comes to predicting building energy performance, one can argue there are "lies, damn lies, and computer simulations."
Computer simulations are complex and expensive. The budget model against which a proposed model is compared is not an efficient solution, but the most inefficient possible. Then, how does one predict the performance of an innovative system for which no computer model exists? Currently, under the requirements of ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, the analyst is required to game the simulation because the algorithms in existing simulation programs may not be able to model innovative systems. This permits false claims of performance.
Two things are needed. One is a design process that leads to the best solution every time. The second is a simple scalar mechanism to measure HVAC-system efficiency, similar to EER or SEER. Such comparisons should be made against a hypothetical best possible outcome, not the worst. Designers need a clear outcome for which to strive.
Characteristics of High-Performance Systems
High-performance HVAC systems share certain characteristics:
- They are designed to eliminate waste and deliver only what is needed, only where it is needed, and only when it is needed.
- Because they functionally eliminate systemic inefficiencies they inherently approach theoretical limits, and their actual energy-use characteristics are easier to compute and more predictable.
- Their designs tend to be physically simple. Heating, cooling, and ventilation functions typically are decoupled and separately provided in ways that permit each to be controlled separately and not interact with the others.
In addition, ventilation subsystems not only must efficiently process outdoor air, but should be able to manage it down to the individual-room level. This is simply, easily, and economically done using 100-percent outdoor air.
Achieving the highest-performing systems will involve the aggressive use of the following technologies, in addition to renewable-energy systems:
- Evaporation/condensation/freezing and thawing. Being adiabatic processes, they do not consume energy, but create thermal benefits through phase changes. This permits us to convert available energy assets from unusable to usable forms. Thermodynamically, these are the only true "net-zero" HVAC processes. They permit energy to be stored, have no carbon footprint, and are 100 percent sustainable. Finally, effective and environmentally sustainable solutions have been developed for all of the "problems" commonly attributed to them.
- Air-to-air heat-exchanger technologies. Multiple types are available. Each type has unique characteristics. Which is best for a given application depends on design goals and how it is to be applied. Multistage air-to-air heat exchangers can be used to create synergy between processes, sharply reducing both heating and cooling loads. When effectively applied, they permit 100-percent-outdoor-air systems to be four to 10 times more efficient than recirculating-air systems. Reductions in ventilation heating and cooling energy use as high as 97 percent, and 80 to 85 percent, respectively, have been documented.
- Thermal and electrical storage. Without some mechanism to store excess capacity, renewable- and recoverable-energy assets must be discarded. Storage permits energy assets of all types to be retained for later use and improve renewable returns on investment.
The Jury is Out
Are cost-effective, net-zero energy building designs possible? For certain types of facilities, yes. For others, the jury still is out.
HVAC systems will never get to net-zero by themselves, but they can be made significantly more efficient. However, getting there cost-effectively means eliminating designed-in waste and integrating building systems to retain previously expended assets for future use.
We cannot get to net-zero with the strategies, systems, and solutions of the past. The HVAC design community ultimately will need to abandon those strategies and learn not only how to design fundamentally efficient solutions, but how to apply the principles of high-performance design. We must move away from strategies designed to expend new energy assets, relegating that option to the resource of last resort. We must learn to operate on what nature gives us for free and what can be recycled whenever possible. For these reasons, the routine application of high-performance design principles will not come as an evolution of existing HVAC design practices, but through a technological revolution in the way we design buildings.
Progress can only come one step at a time and will be built on past progress. Design firms that fail to make adequate progress will wither and die in the coming market. Those who do will survive and prosper. Furthermore, “faking it” will not be an option, and those currently engaging in the practice should expect litigation. Those who choose that path should expect to be held accountable.
1) Lentz, M.S. (2009, January). Regenerative Dual Duct: A case study. HPAC Engineering, pp. 20-26.
Mark S. Lentz, PE, is president of Lentz Engineering Associates and a member of HPAC Engineering’s Editorial Advisory Board. He has received numerous national engineering awards, including a Distinguished Service Award from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.