Our house on the YO Ranchlands is a passive solar home. It is constructed with white limestone which minimizes the heat gain from the sun in summer. The walls are 1 foot thick with 4-6” of stone, a 2×4” stick wall with fiberglass insulation and then wall board inside. The ceiling has R45 fiberglass above it to minimize attic heat in summer and cold in winter. All windows are double pane. Our house sits on a berm of 44 dump truck loads of limestone rich dirt compacted on an old limestone reef. The house operates like a raised bed garden with the berm getting warmer than the soil below it, just as a raised bed garden does. The house faces north since here in south central Texas cooling is required more than heating. This berm, and the thick cement slab the house sits on collects heat over the summer and disperses it into the house over the winter. Monitoring the floor temperature in the house for a year showed that the floor maximum was 86 degrees in late July and minimum 64 degrees in January. Since we like to keep the house at 75 degrees we only have to heat or cool 10 degrees at most and much of the cooling can be done by simply opening the doors and windows at night.
Figure 1. Household floor Temperature (oF) from January 1 or Julian Day 1 to day 365. August 1 is day 214.
SOLAR POWERED HOT WATER
The first solar project we did at the homestead was to put in solar hot water. Our first system was a batch heater with evacuated tubes, that cost about $3000, which we operated for five years. It would boil water most days even in winter, but during unusually cold winter weather we had inflow pipes freeze three times during hard freezes, and decided that we had to move to a circulating glycol system. Our glycol system has a Stiebel Eltron flat plate collector, storage tank, 0.25 hp circulating pump and controller. Except for December through February, it makes all the 130 degrees F hot water we need. In those winter months it takes our 50 degrees well water to 80 or 90 degrees and we have a 220 V PowerStar AE12 booster to take it to 120 degrees. This system cost about $6000 and has been functioning flawlessly for 6 years without any maintenance except washing the dust off the collector once a year. Heating water is one of the biggest uses of energy in any home and I think anyone living in a sunny state should install solar hot water as their first solar project.
SOLAR ASSISTED HEAT PUMP
One active solar feature of our Texas hill country home is a Hotspot ACDC12b solar assisted mini split AC and heating unit. This unique solar air conditioning technology requires no batteries, no inverter, no controller it plugs into the solar panels directly. During the day it runs primarily on solar power plus some 220V power. Hybrid operation eliminates the need for batteries and allows 24 hours per day use of the system although we only use when the sun is shining on our 750 Watt solar panels. The system uses solar power, and mixes it with normal AC power as needed. It runs so quietly we often do not notice it is on. Augmenting our space heating or cooling systems with solar made sense for our passively heated and cooled solar home. Our 1200 square foot home is heated and cooled primary by storing the summer heat in the large earth berm the house sits on, and its thick cement slab. The house has 1 foot thick walls, R45 in the ceiling and double pain windows. Conservation measures like insulation are the best energy saving investments anyone can make. With no external heat or cooling the warmest the house gets in late fall is 82 degrees and coldest 60oF in late winter as measured at the thermostat with is at eye level. We set the mini split unit to 74 to 78 degrees in the summer, depending on what we are doing in the home, and let it run every day. We turn it on when the sun hits the solar panels and turn it off when the sun sets. Ceiling fans help distribute the cooled air throughout the house. The Hotspot mini split unit has reduced our use of the conventional ducted 220V AC to zero. The ACDC12b hybrid solar unit also does a good job of filtering the air which is especially nice when ash juniper or cedar pollen is polluting the air. This has been one of the biggest attributes for us. In winter we set the mini split at 76 degrees and run it whenever there is sun on the solar panels. It easily heats the house unless there is a very strong cold wind and as I said before filters the air as well. On cloudy winter days we use wood heat. The Hotspot solar AC-Heat system makes the most financial sense for new construction or if you have an old ducted system that needs total replacement. If you do not have room to install ducting this is a wonderful technology. Doing what we did and placing it beside a perfectly functioning conventional 220V and Propane ducted heating-cooling system has not been a big money saver, but for us we love the technology and comfort the system provides. Reviewing past electric bills our summer KW consumption before and after installing the solar powered AC is about the same. However, with the older 220V system we only turned it on when the house was 80 degrees, and set the thermostat at 80 degrees, but with the Hotspot mini split we turn it on most summer days and run it until 9pm set at cooler more comfortable temperatures. In late summer we keep 4 ceiling fans running 24/7 to aid the AC. In short, we run the solar AC unit a whole lot more than we ever ran the old 220V system and keep the house cooler. Our house is at 2300 ft elevation on a hillside with a good breeze most days so at night we open all the windows for natural cooling no need for AC.
SOLAR POWERED REFRIGERATION
We installed a solar powered 8 cubic foot SunDanzer DC refrigerator and a 14 cubic foot freezer that live in out in our barn. These high efficiency refrigerators and freezers have exceptionally low energy consumption requiring only a single solar panel each. These units are high quality construction and past users report excellent reliability and long life. The super-insulated cabinets feature 11cm of polyurethane insulation with powdered-coated galvanized steel exterior and aluminum interior. A zero maintenance, brushless, thermostatically controlled DC compressor operates on 12 or 24 VDC using NASA developed technology. Built in El Paso they have a low-frost system that reduces moisture build-up for low maintenance. These chest-style refrigerators and freezers are easy to clean and drain. My system is not gird tied so the 14 cubic foot freezer has a 24V deep cycle battery bank and the 8 cubic food refrigerator one 12 volt battery. After we installed these units and unplugged one conventional chest freezer and an older refrigerator our KW usage was cut by about 30%. This is not surprising since refrigeration is one of the biggest consumers of household electricity. The battery banks stay 100% charged even after powering the units all night. Neither rain or clouds during the day prevent the panels from keeping the batteries fully charged. Likewise summer heat or short winter days do not seem to make any difference, the systems just functions flawlessly with batteries at 100% charge state at sunrise regardless of the solar regimen. In fact since I have a surplus of power I set the freezer to -20 and the refrigerator to 34F, 2 degrees above freezing. Believe me food lasts a long time in that refrigerator than one set at the typical 40F. This investment really saves us money on our utility bills and more important to us we do not have to worry about food spoiling when the grid goes down.
RETURN ON FINANCIAL INVESTMENT
Looking at the dollar savings on my utility bill, it looks like I save about $600 a year over what I paid before adding the solar hot water, heating and cooling and refrigeration, and that is about a 3% return on my financial investment. In terms of kilowatts, though I have cut my energy use in half. None of my solar powered systems are gird tied. I was an early adopter of solar when prices for solar panels were much higher than now, and my utility company did not allow property owners to become power produces. Our utility now encourages homeowners to go solar. At 2017 prices the dollar return may well be much better than my 3%.