South Facing Solar House

The house footprint is 1635 ft2 with the lower level tucked into the hill side.  There is an unheated, 1348 ft2 attached garage on the north side of the house, half of which is for cars and the other half houses my work shop and the PV system electronics and batteries.  Being a bit paranoid about wild fires, I decided to use James Hardy concrete siding and a metal roof.  This would at least give the house a better chance of survival should a wild fire move through our area.  The house uses both passive and active solar heating.  We also use a wood stove for heating and use about 1 ¼ to 1 ¾ cords of aspen and ponderosa pine per year that I cut off of our property.  Our final backup heat source is a propane fired boiler which is integrated with the active solar heating system to provide heat via the radiant floor heating system.  Over the past two heating seasons the boiler has fired up for only about a total of 35 minutes to provide space heating.  I’m estimating it has run for about 10 hours during the same two year period to provide heat for domestic hot water (DHW). 

My wife and I are in our 70s so we keep the house warmer than when we were younger.  The indoor daytime temperatures are usually in the low to mid 70s and the thermostats are set to keep the night time temperature at about 67 - 68 ºF.  Averaged over the past nine years, which is about as long as I’ve been keeping temperature records here, our heating requirements have amounted to 8705 degree-days per year.  Our coldest temperature was -23 ºF on Feb. 2, 2011.  Since our last propane fill up on Oct. 1, 2008, usage has averaged 45 1/3 gallons of propane per year.  We cook with propane, have a propane fired clothes dryer, a Kohler 10RY62, 10 KW propane fired generator (only started and ran for 10 minutes several times each year just to make sure it was still OK), and the boiler (Buderus Model G 124X-18) which, when needed, heats DHW and provides space heating.

Many people still remember the bottle and tire solar homes from the 1980s.  Those were the exception, not the rule, but they got the publicity.  Here are some pictures of the inside of our solar house.

Kitchen

Living Room

Master Bedroom

Woodstove

The outside walls are constructed with standard 2 X 6 lumber and filled with fiberglass insulation.  Before the sheetrock was installed I put up 1” thick isocyanurate rigid insulation and ran a bead if silicon calk where each sheet butted up against the next sheet.  I also sealed every electrical box and any other penetration in the sheetrock with silicon calk.   The ceiling, which has 12” of fiberglass insulation, got a 6 mil polyethylene vapor barrier before the ceiling sheetrock was installed.  The interior walls are of standard 2 X 4 construction but are filled with fiberglass insulation to cut down on noise transmission from one room to another.

The mechanical room contains the 400 gallon stainless steel solar thermal storage tank (insulated with 3 1/2” of fiberglass insulation along with 2” of isocyanurate rigid insulation), the boiler, the 40 gallon DHW, and the controls equipment.  You may have noted the six 4’ X 10’ foot solar panels (three of each side of the three skylights) in the first photo.  These panels are mounted on the south facing (15/12 slope) metal roof.  They are sloped slightly towards the sky lights so as to allow for draining the panels.  This is a drain back system and only water is used as the heat collection fluid.  All pipes must be sloped to allow for complete drainage of the water back into the 400 gallon tank when the circulation pumps turn off.

In order to save electrical energy, all heating thermostats (there are seven zones, three downstairs and four upstairs) are line voltage devices.  This way, there is no 120/24 volt step down transformer to be consuming power 24 hours a day.  All zone valves are therefore 120 volt valves and their end switches switch 120 volt power to the rest of the heating controls.  Therefore, as you can see, all control wiring is contained in metallic flex conduit.  The control system is designed to heat the DHW or space, as needed, first with solar, if the 400 gallon tank can provide the necessary heat, and if not, the boiler will fire up to do the job. 

Solar storage tank, DHW tank, Boiler, and controls

For those of you who enjoy these sorts of things, here is the plumbing diagram of our heating system.  The controls are set up to provide heat for either space heating or DHW, but not both at the same time.  If there is a call for space heating and DHW heating simultaneously, DHW has priority since it can be satisfied relatively quickly.  The mixing valve shown is set at 120 ºF so as to keep the water going to the PEX tubing in the floor concrete at a reasonable temperature.

Heating System Plumbing Diagram

The Electrical System

I chose a ground mounted array simply because I wanted easy access to the back of the array in case of any maintenance that might be needed in the future.  The array tilt can be changed for optimum summer and winter tilt angles.  As shown, the 2680 watt array is at the winter angle.  And yes, I do need to clear the snow pile-up at the bottom of the array in the winter.  I wanted to mount the array higher to avoid needing to do this but due to very rocky soil conditions I wasn’t able to drill a deep enough hole for the west side pier.  Consequently, I felt I needed to keep the piers fairly short and the array close to the ground. 

2680 Watt PV Array

This past April I decided to not change the PV array to the summer tilt angle as I have in the past.  I was curious as to how much less energy I would collect as compared to the average of the previous five summers.  The energy decrease was only 6.1% and since I did not run short of energy during the summer, I’ll probably just leave the array tilt in the winter angle again next summer. 

When we first moved into this house I had installed only 1200 watts (12, 100 watt modules) of Photowatt modules.  My original design for a mounting rack and wire sizing took into account future expansion.  Within a week I knew I was going to need more power so I ordered  8 Kyocera 120 watt modules which gave me a total of 2560 watts of PVs (Photowatt had stopped manufacturing the 100 watt module I had installed).  A couple of years later two of the Photowatt modules failed.  Since the modules are wired four to a series string to give me a 48 volt system, the result was one 400 watt string being out of commission.  So I ordered four Kyocera 130 watt modules.  This gave me the 2680 watts that is presently working like a champ.  By the way, a year or two later I repaired the two failed Photowatt modules.  The problem was that during manufacture of the modules, they had failed to solder the contacts from the cells to the terminal board.  A little solder solved the problem.  I now have four backup 100 watt modules if I ever need them.

I have two Trace SW4048 inverters (4000 watts/48 volts).  One is on 24/7.  The other is set to the sleep mode and wakes up only when needed.  I wanted two inverters for two reasons.  First, I wanted the redundancy, i.e., if one inverter failed, I could quickly do a little rewiring and be back in business.  Additionally, I bought a 120/240 volt step up transformer if ever I needed to operate with only one inverter.  So far, I’ve not needed to go this way.  The second reason for wanting two inverters, I just needed the additional power of the two inverters to run my 240 volt three hp table saw and five hp plainer motors and welder. 

When I first started using this system, I was using two Trace C40 controllers.  After about two years I realized I could operate more efficiently with a maximum power point tracking controller.  So I replaced the Trace controllers with an Outback MX60.  This has worked out great.  I not only get more energy into the batteries, I also have the ability to log the energy collected.

PV Control and Power Conditioning Equipment

The battery bank consists of 32 T-105 Trojan 220 A-H batteries which are housed in my garage in a vented insulated box with a top lid and doors on the front.  The batteries are wired eight in series per string with four strings in parallel.  I had been told not to have four or more strings in parallel and that I would be better off with the L-16 size batteries and three strings.  But I have to say that considering I’m now going into my tenth year with the T-105s, I think they have done better than expected.  Besides, when it does come time to replace the batteries, they are small enough that I can handle them without any lifting help since they weigh about 65 lbs each whereas the L-16s weigh about twice that amount.  I have the MX60 controller set up to automatically equalize the batteries every 10 days.  I also check the electrolyte level every month or so, adding about two to three gallons of distilled water as needed.  It takes me about 30 - 45 minutes to complete this maintenance chore.  Being in the unheated garage, the battery box drops into the low 40 ºF range during the coldest winter months.  During the summer months, I open the battery box lid and doors when the battery temperature exceeds about 70 ºF.

Batteries and Battery Box

I monitor the battery charge level with a Tri-Metric TM-2020 battery system monitor.  Typically after a full day of charging to 100%, I’ll use about 12 % of the battery capacity over night in the winter months.  Maybe once or twice during an extended period of cloudiness, the battery bank will drop to 55 – 60% of capacity.  If the forecast is for a clear day, I’ll let the sun take care of charging the batteries.  If the forecast is for continued cloudiness, I just have to bite the bullet and turn the generator on for an hour or so.  I’ve not needed to do this for at least the past two winters.

In summary, I can truly say that this is the most comfortable house I’ve ever lived in.  It was also a real joy to design and take part in its construction and especially to see how well it has performed from an energy stand point.  To those who say that solar doesn’t work, I just say to go ahead and believe what ever you want and continue paying those high utility bills.  It doesn’t bother me in the least.

Jon Klima retired in Dec. 1994 from Red Rocks Community College in Lakewood, Colorado, where he taught in the Energy Technology Dept. for 14 years.  Before that, he worked on the Titan III Launch Vehicle as an Electrical Engineer at Martin Marietta.  He holds a BS in Electrical Engineering and an MS in Civil Engineering/Building Management Energy Systems.  For information on generic solar heating systems, visit Jon’s web site at www.conifer-solar-consulting.com.

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