The sheer joy that comes with creating something marvelous, from scratch, generates an upbeat and infectious mood that’s impossible to avoid when visiting the Solar Decathlon's student teams and their faculty advisors/mentors.
This year's Solar Decathlon held in the District of Columbia was relocated from the National Mall to the Tidal Basin, which made it a bit harder to find and created some interesting adventures courtesy of cab drivers who evidently were also from out of town! Nonetheless we managed to get to the event and thoroughly enjoyed every moment.
Each of our four visits to a Solar Decathlon (held once every two years) has revealed emerging trends and patterns from zany and outside-any-box construction seemingly brought to life from a dream to down-to-earth practical homes you could easily see yourself living in quite comfortably.
Up until this year's competition (between a maximum of 20 teams selected by the U.S. DOE), there was no limit on what these solar energy independent homes would cost in a real-world construction environment. As a result, we witnessed Solar Decathlon homes whose cost exceeded a million dollars to ones quite practical that stood virtually no chance of competing in the juried contests. Witnessing a team being able to buy first place with a virtually unlimited budget was disappointing. To level the playing field, this year's contests (10 competition areas worth 100 points each) included "affordability" and any home costing in excess of $250,000 would suffer lost points. Exceed $600,000 and you would be awarded a big fat total of zero affordability points!
Unintended consequences
Affordability, as a juried contest, not only leveled the playing field, it had a dramatic impact on several things. Solar PV arrays were noticeably smaller in order to minimize the homes’ costs and much more realistic with arrays more closely matching the average annual needs to render each a zero-energy home, on average with average seasonal weather conditions, meaning more days with sunshine than extended periods of cloudy/rainy weather. Unfortunately, Mother Nature did not cooperate and most of the homes were drawing some portion of their required energy from the DC grid, which reduced their points for energy production. In previous competitions where huge PV arrays were the norm, excess power was generated even during adverse weather conditions. Now that affordability has caused the PV-arrays to be more real-world realistic, it may be time to readjust the energy category auditing process to grade the teams on an adverse weather curve, so that it is still possible to have an economically balanced approach to PV system sizing based upon annual insolation averages for the home-based-location of each team.
Gone too, for the most part, are wildly exotic heating and cooling systems that stretched the boundaries of traditional designs. Looking back, the first decathlon we attended included hydronic radiant heating in virtually every one of the twenty homes. Heating was powered primarily by solar thermal collectors with large storage vessels that doubled for the domestic hot water resource. Air conditioning systems were central and ducted with a few being driven by adsorption or absorption chillers. In 2005, we saw just a few mini-splits being used for air conditioning. As each Decathlon passed we witnessed a migration towards mini-splits with a few using newer-style inverter heat pumps along with hydronic radiant floors in 2007, however, mini-splits were still primarily used for cooling purposes. In 2009, inverter mini-split heat pumps were used exclusively for heating and cooling in a handful of the students’ entries while hydronic radiant heating continued its reign in the majority of the decathlon homes. 2011’s affordability contest had an unwelcome consequence: hydronic radiant heating was virtually non-existent. In its place, almost every one of the Solar Decathlon homes had incorporated inverter mini-split heat pumps for their heating/cooling, and in four cases, generating domestic hot water in combination with solar-thermal or via inverter heat pump only!
Death of hydronics is exaggerated
Before we dig a grave and build a cross to honor the memory of hydronics, I believe we witnessed its hybrid rebirth. Not one, but four homes (21% of the homes in the competition) incorporated hybrid-systems using Daikin Altherma air-to-water inverter heat pumps to heat and cool via hydronic coils in air handlers, radiant floors, and provide domestic hot water using indirect water heaters. Given that the Daikin Altherma is a new product launch in the USA and one of the newest inverter mini-split technologies, I had to know how/why they had discovered and chosen the air-to-water mini-splits. (We recently installed our first Daikin Altherma for a radiant heating/cooling project and I knew first-hand that a higher level of skills is required to successfully marry all the connected-systems for harmonic operation and delivery of comfort while designing the connected-systems, so that peak efficiencies can be obtained.) Like any heat pump system, designs utilizing lowest-possible water temperatures, for heating, or highest-possible water temperatures, for cooling, will provide the best COP (coefficient of performance) and lowest operating costs. To be continued in next month's column…
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