Heliospiti at 5 Years: Lessons Learned

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Jim and his boys today. After five years in their passive solar, solar thermal, and PV-powered home, the Riggins family is assessing their home’s systems and their solar lifestyle.
Jim and his boys 5 years ago.
Twelve-inch-thick walls slightly block solar gain at the south-facing windows.
Pulling back a throw rug reveals the degree of fading on the cork flooring due to ultraviolet light exposure.
At winter sun angles, most of the thermal mass floor is exposed to sunlight.
Three SunEarth 4-by-10-foot solar thermal collectors provide a substantial portion of domestic hot water needs, even during cold and cloudy periods.
The substantial temperature difference between the 40°F incoming well water and 120 gallons of 170°F stored solar-heated water required replacing the original 4.4-gallon pressure tank with a 10.3-gallon unit.
The solar thermal differential controller’s high-temperature setpoint was lowered from 205°F to 190°F to prevent vaporization at the 7,000-foot elevation.
Before the addition of a second electric car (purchased in August 2016), the 20-module PV array provided an average surplus of 4.8 kWh per day.
The Enphase Envoy monitoring screenshot shows the lower performance of three modules (dark blue) and complete failure of three microinverters (gray). Possible solutions include a piecemeal or full-array upgrade to more reliable microinverters or switching to a string inverter.
A water-to-air heat exchanger was planned to move heat from the solar thermal system to the input of the HRV air handler. However, passive-solar direct-gain performance is so good that the system was never hooked up.
A water-to-air heat exchanger was planned to move heat from the solar thermal system to the input of the HRV air handler. However, passive-solar direct-gain performance is so good that the system was never hooked up.
A push-button switch in the bathroom activates a pump that circulates hot water through the distribution system to avoid wasting water.
Even on cold days (here, the weather station shows 18.5°F outside), the passive solar gain can raise interior temperatures to comfortable levels (77°F).
At high altitude, a lower boiling point increases cook time and thus energy use. Using a pressure cooker on the induction range shortens cooking time, saving electrical energy.

With the December temperature plunging to -11°F and snow outside, our central Colorado home’s interior temperature was a comfortable 71°F, and rose to 76°F as the storm passed and sun came out in the afternoon. Even after five years in our net-zero energy Heliospiti (“Sun House”), my family and I still marvel at achieving these temperatures—with no mechanical heating.

In two previous Home Power articles, I described the design (HP141) and one-year performance (HP150) of Heliospiti. With five years of experience, this article answers the common question I’m asked when showing the house: “Knowing what you do now, what would you have done differently, if building today?”

The house was built with the Passive House focus of extreme airtightness and super-insulation. Passive solar gain satisfies the bulk of the space-heating load, while a combination of extreme efficiency, plus active PV and solar water heating (SWH) systems meet the full energy requirements for this all-electric house and our electric vehicle.

An all-electric net-metered house makes it easy to determine net consumption and production. Based on the initial meter reading upon moving into the house, and the reading on the five-year anniversary in May 2016, the net excess production of our 4.5 kW PV system was 8,829 kWh, an average of 4.8 kWh of surplus electricity per day. This includes powering the house—plus refueling our  Nissan Leaf since April 2012. The house did indeed perform as a net-zero energy home—with energy to spare.

Passive Solar Resolutions

The passive solar plus Passive House design has met or exceeded our computer modeling results. Even with temperatures as low as -25°F, we put less than five hours on the air-source heat pump during the first five years. It’s clear that the three concepts of ultra airtightness, super-insulation, and passive solar design (which includes ample interior thermal mass) will achieve high heating and cooling performance. All three elements work in harmony to capture winter solar gain, hold it within the house at night and on cloudy days, and provide comfortable, even temperatures throughout the house without the typical cold spots or drafty rooms.

The first passive solar lesson learned was failing to account for 12-inch-thick walls limiting the effective aperture of the south-facing, solar-collecting windows. The thick window wells narrow our solar exposure, reducing the amount of sunlight reaching our concrete slab floor and 1 1/4-inch-thick gypsum wall thermal mass. This is especially a factor with low sun angles early in the morning and late in the afternoon. This solar energy is not lost but it strikes the window well material, not the high thermal mass areas. Building today, I would replace the oak window trim with drywall. Although oak has an 85% greater specific heat value than gypsum drywall, the gypsum is 116% more dense, giving it greater thermal mass value.

Another advantage to removing the wood window trim relates to our choice to use low-iron glazing in the south-facing windows. Although this glazing results in a higher solar heat gain coefficient (SHGC), which is fantastic for improved solar gain, it also increases ultraviolet (UV) transmission by approximately 25%. This UV transmittance created significant fading on the wood trim, cabinets, and second-floor’s cork flooring. The fading was so significant on the window trim after just one year that I had to sand and refinish the trim using a UV-protecting exterior polyurethane.

Water Heating Solutions

Our nonpressurized, drainback SWH system has worked well in spite of reluctance and warnings from the local installers we spoke to during our design phase. When properly installed, nonglycol-based drainback systems do indeed have a very low risk of freezing in cold climates. We like the simplicity and lower maintenance of the simple drainback system, and avoiding the need to build a dump circuit to extract excess summer transfer-fluid heat.

Our key problem with the SWH system, which was easily remedied, stemmed from my lapse in basic high-school-level science: At 7,000 feet elevation, unpressurized water does not boil at 212°F—it boils at 198°F. We initially programmed our Caleffi iSolar Plus controller to shut down the circulation pump when the collector temperature reached 205°F. But this allowed the heat-transfer fluid—distilled water—to flash boil, creating violent vibration in the copper pipes. It sounded as if the system was ripping itself apart in our attic. Air was getting trapped near the circulation pump and bubbles were moving up to the roof. As the transfer fluid heated and expanded, it compressed the air pockets so that when they shot into the drainback tank, it was at high pressure and vibrated the pipes. The key was to bleed the system both high (at the drainback tank in attic) and low (inlet to circulation pump) after the distilled water was partially heated from the sun. Bleeding air from the system and setting the upper shutoff temperature to 190°F solved this problem. The lower shutoff temperature reduced the efficiency of our SWH system by sometimes shutting down on clear days before the storage-tank water reached its maximum temperature. However, it was still an acceptable trade-off for us to have the benefits of a simple drainback system.

Comments (10)

Peter Gruendeman_2's picture

My question is less for the author and more for someone who designs PV systems. When I took the MREA PV design class earlier this year, DC optimizers were presented but the economics were not explored. The author's situation suggests we look at the economics of optimizers. The maker's website explains that they increase the electric output of the panels by up to 25%, especially on shade challenged sites. One seller's price for the SolarEdge P320 optimizers was $80. When used on a 320 Watt PV panel, this will allow an increase of up to 80 watts (25% of 320) for $80., or put another way, possibly for as little as $1/ watt; higher cost (less benefit) on sites that are not challenged by shade.
Why would I pay $1/ watt (or more) when retail for single crystal PV panels is $0.74? Add in racks and we're still looking at $1/ Watt, worst case, vs $1/ Watt gained by optimizers, best case. I wonder if the economic window for DC optimizers has already passed. Maybe a system designer can explain this to me.
Pete Gruendeman La Crosse, Wisconsin

Justine Sanchez's picture
Hi Pete, Great question! The exact economics of how much extra power you will get with optimizers will be site specific, such factors as partial array shading, varying module manufacturing tolerances, module temperature differences, varying levels of module soiling, and so on...so it is very hard to get an estimate of how much exactly you can gain. However consider the fact that the SolarEdge inverter is less expensive than other GT inverters of the same capacity, you need to look at the whole package. I found for my system that I put in about 3 years ago, it was only slightly more expensive to buy the SolarEdge (SE) inverter plus optimizers than a different string inverter of the same size (w/o optimizers). This is because some of the electronics that are in the optimizers are not required in the SE inverter. Then you need to look at what else you get in the SE system...the big two in my mind are module level monitoring (which I find super helpful in identifying individual module problems that would likely otherwise go un-noticed) AND built-in module-level rapid shutdown capability. Rapid shutdown is an add on that you can purchase with other string GT inverters, but is not included and it is only array-level. However in the not too distant future we will likely be looking at module level control being required by the 2017 NEC article 690.12 in 2019 (see our article on Rapid Shutdown requirement in HP 175). Best, Justine Sanchez Home Power Magazine
James Riggins's picture

Article Update (Aug 2017): Four more Enphase M190 microinverter failures, and the inability to get the more reliable Enphase M215 inverters as warranty replacements, led to a difficult decision. I replaced all M190s with 20 SolarEdge P320 power optimizers, and a transformerless SolarEdge SE5000H HD Wave string inverter.
The crew from El Paso Green Energies in Colorado Springs did a highly competent and professional installation, and were a joy to work with.
Although payback time will be lengthy, I expect improved performance of the new system beyond just the increased reliability. The SolarEdge optimizers will allow full power output from the modules, whereas the Enphase M190s would clip my 224 watt modules at 199 watts on ideal days. Additionally, the SE5000H is rated at 99% efficiency, versus the 95% efficiency of the M190s.

FrankieB's picture

Hi, great project! For a limited back-up supply from PV when the grid is down without having to deal with batteries, check out "Secure Power Supply" for Sunny Boy inverters. The 2 kW you can get is probably not enough for the well pump, but should cover your other loads when the sun is out...

Mark and Linda Call's picture

I haven't read your other 2 articles yet, and maybe this was answered there, but how do you keep the air in your house at acceptable clean levels with having an ultra airtight house? Do you have an air filter going in the winter?

James Riggins's picture

Mark and Linda, you are absolutely correct that an air tight house needs mechanical ventilation. We use an energy recovery ventilator (ERV) by UltimateAir called the RecoupAerator (www.ultimateair.com). Our intake air comes through a 10 foot deep, 100 foot long earth tube where the gound temperature is a constant 49 deg (F) year-round.
This model has a sensible recovery efficiency (SRE) of 83% meaning that 83% of the outgoing (exhaust) heat energy is transferred to the incoming cool, clean ventilation air. Another feature we like about this model is that is has an EconoCool mode. In the summer, turning on this mode prevents the heat transfer from occurring so that the incoming ventilation air is at approximately 50 to 55 degrees. We don't run it much in the summer except when strong winds or thunderstorms prevent us from opening windows. This unit has been trouble-free since installation, and filter cleaning is very easy.
For energy savings, you don't want to install a higher than needed ventilation rate. To determine the correct ventilation rate for residential buildings, use the national standard, ASHRAE 62.2 from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). A typical house will only need between 65 and 75 cubic feet per minute, but this will change for very large or very small homes.

Carl Wagner_2's picture

Hi: Great to read your article. I just got SWH myself,everything much smaller than your scale. As for losing electric when the grid is down, believe someone will solve this within 10 years - it's so useful to solar generation. In the meAntime, why not install a manual transfer switch and buy a hybrid. By reading HP (subscription now expired :-( ) I learned connections to the standard battery can support 900w and to the drive battery are good for about 3000w. Even in your electric car, there is a large battery, though no charger for the battery. Cheers

James Riggins's picture

Carl, we have now gone gasoline-free and own 2 electric vehicles. Being able to use these for emergency backup power is highly desirable to us, but unfortunately, is not yet an option in the U.S. HP #173 has a great article on the status of using an EV to power a home.
The issues are the lack of a utility standard in the U.S. to connect EVs to homes to prevent grid instability, antiquated grids in some parts of the country, and the lack of hardware to get 2-way flow of electricity from a J1772 (240 volt) or CHAdeMO (480 volt) port on an EV.
There is extensive work and progress in Japan on this issue and we can't wait until it shows up here!

Peter Gruendeman_2's picture

Hi:
I agree that backup electricity for running one's well pump would be nice-- make that a necessity, especially if the power is out for more than about 24 hours. You stated that your well is 640 feet deep so it's no easy task to run that pump on backup power. I fear your pump's starting surge (current) will be quite high and that it will draw lots of power when running. Batteries store Energy; inverters dispense Power. You won't necessarily need a large battery bank but it will be required to operate at high rates. Or buy a larger battery bank and take care of it. Batteries suck. Being without the well pump sucks more. Been there, done that. If possible, measure your pump's starting surge before you shop for an inverter. You are likely to need a powerful one (7-10kW?) just to handle the starting surge.

I was surprised to read that your caption stating that the solar thermal DHW system provides a substantial portion of your DHW needs, even during cold and cloudy periods-- surprised because the SRCC data on the SunEarth EC-40-1.5 shows they produces about nothing under those conditions (Category D). Then I realized that with your very cold well water, you're probably running at Category C or even B, so that very cold water entering your collectors is in fact coming out less cold. That's not suitable for showering but it is still energy collected and reduces your need for purchased energy.
Request for opinion: Another time around would you install solar thermal for DHW or would you provide DHW with PV? Some say the complexity and cost of solar thermal is no longer justified on the small scale because PV is so cheap.
Pete Gruendeman
La Crosse, WI

James Riggins's picture

Peter, let me answer your questions out of order because I think your last question is the most interesting.
1. Would I go with SHW if designing today? The answer is no, because the economics has changed dramatically over the 8 years since I designed my house. If I were building today I would add additional PV modules and install a hybrid electric water heater, my favorite being the Stiebel Eltron Accelera family of water heaters. The reasons are a) PV costs are now 1/6th of where they were when I designed, b) excess SHW energy in the summer is wasted, but excess PV energy can be used elsewhere in the house or sold back to the utility, and c) increased reliability and decreased maintenance.

2. SunEarth collector performance. I designed using the SRCC "Mildly Cloudy," Cat C data for the SunEarth EP-40 modules. The SRCC data shows 21% efficiency at 90 deg delta-T (F), and a Cat C output of 29.7 kBTU/panel/day. The SRCC data turned out to be pretty accurate in our design. But the other critical design aspects, beyond instantaneous output, are storage and use rate. In Colorado we rarely go more than one or two days between clear, sunny skies so having a thermal "battery" that covers 3 to 4 days of use is sufficient. We get that with only 120 gallons of storage because the WaterSense plumbing fixtures, hot water recirculation, and ENERGYSTAR water-based appliances drive our consumption to very low levels. Our hot water rarely gets below 120 deg (F) and I think the lowest we've ever seen is 115 deg after a multiple days of continuous fog. Finally, when I discussed in the article about the SHW system kicking on in cloudy or foggy weather, it is based on the differential temperature between the collectors and the water in the bottom of the storage tank. Bottom water may be down to 95 degrees, but we draw hot water from the top at 140+ degrees. So yes, the SHW system on a cloudy day may only be providing 110 degree water to the bottom of the tank, but we are still getting sufficient hot water for showers, dish washing etc.

3. Well pump backup. Since writing the article, I started detailed analysis of the well pump to price out a solar backup. I connected a high sample rate Hobo data-logging clamp ammeter to the pump controller to measure current and power spikes. Our 3-phase, soft-start variable speed pump (Goulds 7GS with Aquavar Solo controller) performed better than expected. The largest spike was only 4.1 amp and peak power was 996 W. This was easily addressed with a reasonable size battery backup. However, when looking at the relatively few times we've lost grid power, and mostly for very short periods of time, we could not justify the cost of adding battery backup. One side note: after researching numerous battery type options for cost, safety, and longevity, we opted for the 48 volt "salt water" battery from Aquion Energy. This battery cannot experience thermal runaway, and can be discharged to very low levels for greater cycles than Li, lead acid or Ni batteries. The week I was going to contact their distributor for information, the company declared bankruptcy. I hope they restructure and continue to provide this promising technology.

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