Jonathan Woolley Talks with the Cooling Connection About Mechanical Design for the Honda Smart Home
We will continue to advance the technologies and integrated design strategies to help the zero net energy vision become a practical and cost effective reality.
Deep energy savings for heating, cooling, and ventilation in homes requires more than installation of a “high efficiency” option air conditioner. While upgrading to the higher efficiency conventional air conditioner may reduce energy use, major energy savings cannot be fully realized unless the many aspects of thermal energy waste in a home are addressed. A whole building systems approach should first reduce the need for heating and cooling (through passive architectural design features), then minimize thermal losses from systems and recycle waste energy where possible. Finally, the remaining heating and cooling requirements can be satisfied with an efficient mechanical system. This approach allows for smaller mechanical systems, and enables the effective use of systems with much higher efficiency.
Jonathan Woolley of the Western Cooling Efficiency Center (WCEC) is one of the lead engineers responsible for the vision, design and implementation of the thermal energy strategies at the Honda Smart Home. Woolley is an advocate for a holistic approach to systems design that targets load reduction and waste heat recovery in combination with efficient mechanical equipment. In close collaboration with Davis Energy Group (DEG) and American Honda Motor Company, Jonathan advanced a thermal systems design for the Honda Smart Home that reduced heating and cooling loads by more than 50%. The approach leverages passive solar design strategies, natural daylighting techniques, a highly insulated thermal envelope, and an advanced sealant technology developed by UC Davis that reduced envelope leakage to 75% lower than IEEC standards.
After reducing loads, WCEC and DEG developed a mechanical systems architecture for the Honda Smart Home that takes advantage of natural daily temperature changes, minimizes losses for thermal distribution, and recycles waste heat. The result should require less than half the electricity input as would be required by traditional systems in a similar home. The unique design utilizes radiant heating and cooling in combination with a reversible ground-source heat-pump that provides all of the heating cooling and domestic hot water for the home. The system recovers heat from warm drain water, and uses waste heat from the cooling process to pre-heat domestic hot water. While this is all very promising, are these technologies economically viable? Can we hope to see these savings for production homes in the near future? We asked Jonathan Woolley to elaborate on the possibilities.
What are some of the challenges that you and Davis Energy Group wanted to address?
One big challenge we targeted with this project was the economic viability for ground source heat pumps (GSHP). It is well understood that ground source systems can be more efficient than air source systems, but first cost for the ground coupled heat exchangers is typically prohibitive. Conventional vertical ground heat exchangers require drilling hundreds of feet into the earth. The equipment and expertise required to execute this can be very costly, and efforts to avoid contamination of deep ground water aquifers can be onerous. It could easily cost $20,000 to install traditional ground source heat exchangers for a home. The approach we are piloting with the Honda Smart Home is far less expensive. We installed a helix of tubing into a 24 diameter 20 foot deep bore. The auger equipment used to drill such a bore is much less expensive, and regularly used to install structural pilings or utility services. The approach does not drill through bedrock and does not risk contamination of ground water aquifers. The installation is quick, a two man team could easily install heat exchangers for a home in a single day. Realistically, this approach could reduce the cost of a ground heat exchanger array by 80-90%.
How will this new wide bore helical ground heat exchanger perform compared to a traditional deep vertical ground source system?
We are now monitoring these systems very carefully to evaluate their performance. Since these wide bore heat exchangers reside closer to the surface, and pack tubing into a tighter space, we expect to find some performance penalty compared to the traditional strategy. There are some reasons to expect that the penalty may not be significant, and we certainly expect it to outperform an air source system. The upshot is that a minor loss in performance is greatly out-weighed by the major cost savings. It should be noted that a number of manufacturers are advancing related strategies such as horizontal trench systems and horizontal directional drilling. We expect the wide bore helical heat exchanger will cost less and outperform these alternatives. Our formal results for heat exchanger performance and will be published later this year.
What other technologies did you choose for the Honda Smart Home? Why?
The heart of the thermal energy system for the Honda Smart Home is a reversible water-to-water heat pump. This single machine provides all of the heating, cooling and domestic hot water for the home. Heating and cooling is delivered through a radiant floor and radiant ceiling. Heated water from the heat pump can also be directed through a storage tank to indirectly heat domestic hot water. When the system generates cold water for cooling the home, waste heat from the system is used to heat domestic hot water. When hot water drains from the shower, tub, or washer, it flows through a heat exchanger and transfers heat off to cold water at the inlet to the domestic hot water storage tank. One key goal for design of the Honda Smart Home was to demonstrate the application of commercially available technologies that promise to simplify the overall mechanical systems for a home. Currently, some of these technologies cost a premium, but in time we expect they will be less costly than traditional residential mechanical systems.
Do you really think that ground source heat pump can be less costly than a standard split system?
In time I expect this will be the case. The current cost of these strategies is driven by low volume production, and by lack of familiarity amongst builders and mechanical contractors. The rate of adoption and market penetration drives overall economies of scale. The recent trend in cost for photovoltaics is a clear example of this fact. Generally, we can estimate the mature market cost for a product by considering: 1) material requirements, 2) manufacturing complexity, and 3) installation time. When you compare a heat pump to a standard split system, the overall materials cost is actually less, and manufacturing complexity is at parity. Installation of these systems should be less burdensome as well, though there are currently significant needs for building contractor familiarity and advancing technical elements that would ease installation challenges.
Beyond cost for the heat pump machine alone, the design strategy we’ve advanced for the Honda Smart Home eliminates the need for ductwork, and combines multiple system functions into one machine. This effectively replaces three pieces of mechanical equipment (outdoor condenser, indoor air handler, and hot water heater) with just one (multi-function heat pump) plus a hot water storage tank. Consolidating this equipment into one machine enables efficiency improvements, and waste heat recovery. Lastly, it should not be overlooked that the envelope and other load reducing strategies for the home allows for a much smaller mechanical system, which is a considerable cost reduction measure.
What is your end goal for this research?
I’ve been very happy to work with Davis Energy Group and Honda to develop such a savvy mechanical design for this research home. Ultimately we want to work with other builders and developers to continue developing and simplifying the design features and control strategies that we’re piloting at the Honda Smart Home. In time the efficiency elements we are demonstrating will become more accessible for the average home owner. To be fair, the Honda Smart Home is not the home that every developer could build in 2014. The project demonstrates the low energy home of the near future. There is some way to go between now and that future, but we will continue to advance the technologies and integrated design strategies to help the zero net energy vision become a practical and cost effective reality.