Mon, Dec 18, 2017
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Arizona Solar Center Blog

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Living with the Sun in the Desert

In the desert south-west the intense sunshine and long summer days result in uncomfortable and even dangerously high temperatures for about four months.

There is little relief at night in the urban areas due to a growing heat island effect produced by a combination of dark tile roofs, dark structures, asphalt roads, parking lots, etc. that slowly release their built up heat at night. Comfortable living requires affordable air conditioning of buildings, not just during the day but also into the nighttime hours.

The advent of air-conditioning some 115 years ago allowed growth in the hotter southern states as a year round option.   Air-conditioning requires a lot of energy, but over the last 60 years the inflation adjusted cost of energy has decreased and the efficiency of cooling has increased, resulting in making living in the desert south-west relatively more affordable, but more energy intensive.   One effect has been that the typical size of residential electrical service panels for buildings has increased to accommodate these higher loads. Larger and multiple TV €™s, Air conditioning, electrical space heating,   larger buildings, and "convenience"?   items like stereos, pools, hot tubs, and microwave ovens have become common place features in homes.   Today 200-amp service panels are common place to meet residential needs, in the future many homes will need to have 400-amp service panels.   This will be driven by a need for high power wall chargers (HPWC) for one or two electric cars, and an option of a solar array on the roof with a battery bank.   To meet the growing electrical demands houses need to be smarter to maintain the electrically demanding lifestyle we take for granted at an acceptable cost.

When I built my south-west dwelling I wanted to be able to have these options without putting an unreasonable and expensive load on the utility grid.   One common feature here that we opted out of is a swimming pool that is utilized for only around 1/3 of the year, instead we opted to put in a garden.   Our home is 3,480 sq ft., it is all electric with no gas onsite, has city water with septic, has a hot tub, a chest freezer that is indoors and not outside in a hot garage.   The indoor summer temperature during the summer months is in the 78 °F to 79 °F range.   An 11-kW photovoltaic array on the roof produces a surplus of around 3,000 kWh on an annual basis.

When building in a climate like this, one cannot understate how critical the orientation is in building a structure. To this end the design had a north to south exposure with the north side being the entrance.   The south side has a shaded patio from end to end with an overhang that allows the sun to hit the south wall only in December and January.   The East wall has two minimal windows that are small in size; the East side also holds the water heater in an unconditioned closet to minimize the cooling load.   The West side has no windows with the garage added as a buffer to the conditioned part of the house, the West side also has a set-in porch to add shading for most of the day during the summer months, in addition there are trees and storage that limit direct sunshine on that portion of the wall.   The Northside has a setback with the garage shading a good portion of the wall, there are trees, and the front entrance never sees sunlight throughout the year.   This thermal decompression helps to control extreme temperatures from the direct sunlight.

The exterior of the building uses lighter colors on the walls as well as the roof, this reflects sunlight to reduce heat absorption.   When designing a building keep the simple thought in mind that heat migrates to cold.   If you have an air-conditioned structure at 78 °F, and the outside air temperature is 115 °F, you have a dark surface that can reach 170 °F. All that heat is trying to get into the cooler structure.   The windows were all recessed around five inches as well as the overhangs, this limits the amount of direct sunlight entering through the least insulated part the of the structure, the windows.

The building itself is CMU block (a standard size concrete rectangular block used in building construction) that had all the cavities insulated with blown foam insulation.   Then a traditional wood structure was built inside the CMU walls, this is what allowed the windows to be recessed.   The wood wall then also had foam blown into the cavities between the studs, this helped to make the home quieter to the outside noise and gave added insulation.   The structure has light colored stucco on an eight-inch insulated block, with an insulated 4-inch wood followed by the drywall interior, for a total of around 12 inches of depth.   The ceiling in the attic was also spray foamed creating a conditioned space in the attic.

The chest freezer is in the conditioned area for higher efficiency.   Lighting for the most part is LED with some CFL and incandescent lamps.    Ceilings range from ten to fourteen feet throughout, this is to open up the space and allow more sunlight in.   There is a SunPower 11-kW solar array on the roof, facing south at a 22-degree pitch that is not obstructed, giving optimal sunlight year-round.   The array is two rows of modules, kept 6 inches off the roof to allow for air flow and cooling.     There is a drain-back style solar hot water heater that has the backup heater breaker turned on only from late November to February.   The water heater also has a timer to allow solar gain during the day and limits any electrically heating of the storage tank to off-peak? hours.

The cooling load is the biggest electrical load for a home in the desert south-west. During the spring/fall shoulder months we turn the HVAC off and during the winter only minimal morning heating is needed.   It is the summer months where it gets interesting, with the strong insulation on the home we were able to get away with a 5-ton HVAC for cooling or around one ton per 700 hundred square feet.   Taking it a step further for the last six years we have used a Hydron Geothermal unit that requires less energy than traditional HVAC units.   Due to the dry earth in the desert, the system needed two trenches for a totaling 500 feet, with 1000 feet of water filled piping inserted into the trenches.   This allows the captured heat to migrate into the cooler ground.   A cooling tower the size of a traditional HVAC unit will supplement during the hotter more humid part of the summer to add some assistance.   As a note when the home was built this geothermal unit was around the same price as putting in multiple air conditioning units.

To add to the quality of living we have hens giving us free range organic eggs, and a garden with tomatoes, peppers, lettuces, radish, carrots without the pesticides and herbicides so prevalent in today's farming practices.

Overall we have been pleased with how it all came together creating a home that functions properly and is very comfortable to live in without having to pay a high electric bill.

What would I have done differently to this house along with things that still need to be done for this to be a complete home are;   First, as always seems to be the case is the need for more space with larger closets, a larger office, and a bathroom attached to the guestroom.   More practical changes would be to have a grey water system for the property, this would lessen the load on the septic system and bring moisture to the trees in a very dry environment.   More shade all around the house and garden, it is amazing how much cooler the shaded zones are vs. the non-shaded ones.   This would include a deeper porch along the southern exposure.   The sun does not hit the south facing wall until December, but in September to November the lower sun still makes the back porch sunny, an extra 4 feet or so would solve this.   The house gets a lot of natural lighting but some of the windows realistically could be smaller allowing for more insulated wall area.   Installing a fresh air intake, this would be utilized during the cooler nights from the shoulder months of September into December, and then March into June.   At some point adding another couple kW of solar capacity with a battery bank could allow the home to function in the case of an extended power outage during the summer months. Lastly, and it will be fairly easy to pull, is the electric for the HPWC's to the garage for the electric cars.   This in turn will call for an upgrade on the 200-amp service panel to 400 amps.  

This completes the home going forward as a sustainable happy place that should be a model for a happier lifestyle.

Geoff Sutton

 

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