Solar energy at home

Most of the energy we earth bound humans consume comes directly from the sun, exceptions being atomic fission and some types of chemical reactions.  Fuel oil, coal and natural gas energy that civilizations use exist because of the Sun’s previous contribution in the formation of those hydrocarbons.  Wind currents are caused by the sun warming the air and as thermals rise they are displaced by denser, colder air.  Likewise the sun’s energy is ultimately responsible for distributing snow melts and rainwater water to higher elevations, which create the kinetic energy needed to power watermills and hydroelectric generators.  On a small personal scale, more individuals are learning to exploit the sun’s energy to heat their homes, generate their own power or to cook their food.  The two main methods of acquiring power from the sun are photovoltaic (PV) cells and thermal energy collectors.

Almost 53% of the energy in sunlight is absorbed or reflected before it even hits the surface of the earth.  The glazing or protective substrate in a solar collector can further diminish the amount of energy obtained.  Even the best solar panels can be considered to be inefficient.  The amount of energy collectible by a given solar panel is subject to many variables.  Whether talking about heat or electricity we generally measure that energy in units of Watt-hours (energy = power x time).  Under the best and brightest conditions a panel might collect as much as 2,000 Watts per sq. meter but under realistic or averaged conditions the expectation might only be half that.  During the daylight hours of a normal summer day at 40 degrees latitude, a solar collector would be doing good to average 600 Watts per sq. meter.  In wintertime for the same location the same collector might gather an average of only 300 Watts per sq. meter.  For any random location around the earth the average collectible solar energy per mean solar day (24 hours) is only about 164 Watts per square meter.

Overview of PV

In a photovoltaic solar cell an electrical charge is generated when photons excite the electrons in a semiconductor.  There are many types of solar cells and even some new developments in technology which will hopefully lead to the future manufacture of more affordable photovoltaic solar panels.  The warmer the photovoltaic solar panel gets the less power it can produce.  Essentially the temperature doesn’t affect the amount of solar energy a solar panel receives, but it does affect how much power you will get out of it.

The most common photovoltaic solar cells are made by chemically ‘doping’ a very thin wafer of otherwise pure monocrystalline (single-crystal) silicon.  In a delicate and complicated process of fabrication, wafers of silicone are generally cut or sliced as thinly as possible (before they crack) to a thickness of about 200-micrometers or the width of a typical moustache hair.  Since each individual solar cell produces only about 0.5V, several cells must be wired together to produce a useful photovoltaic array.  Mostly produced in China, commercial photovoltaic solar panels are very expensive, averaging $2 – $3 cost for every single watt they produce.  An average U.S. residence consumes something like 30.6 kWh per day, 920 kWh per month or 11,040 kWh /year.  In a country like the U.S. where grid power is comparatively cheap (averaging 10 cents per kWh in 2011) it would take a very long time for photovoltaic panels producing equivalent energy to pay for themselves.  In the meantime an individual with a “do it yourself” mentality can more directly utilize solar energy by fabricating his own contraptions to collect heat.

Solar Ovens

Although it would not be considered a quick process, it is easy to cook food with direct sunlight.  Slow cooking oftentimes creates superior dishes with the best blend of flavors.  Some heat trap type solar ovens can easily produce temperatures over 250 deg F; sometimes up to 350 deg F.  No matter what type of oven is used however (electric, gas, solar, smoke pit or Dutch) a good cook knows that slow cooking with a modest heat over a long period, will make an otherwise tough piece of meat more tender.

Essentially there are only two types of solar oven; those that entrap heat or those that reflect it.  To form a simple ‘heat trap’, a cardboard or wooden box can be insulated, spray painted black inside and then lidded with glass or clear plastic.   It helps when the cooking vessel itself is dark also – to better absorb solar heat.  In addition to being dark, it helps when pots are thin and shallow and have tight fitting lids.  Even glass mason jars make useful solar cooking utensils.  These can be spray painted black and the lids can be unscrewed a bit to allow vapor pressure to escape.   It might seem that parabolic or concave reflecting cookers would be complicated to construct, but some examples have been made by simply surfacing the inside of umbrellas or parasols with aluminum foil.  Mirrored Mylar or similar BoPET films are also useful materials in this type of application.  Doubtless many examples or ‘instructables’ detailing the construction of reflective type solar ovens, exist elsewhere on the Internet.  Some specially constructed reflective ovens claim to be able to reach temperatures of nearly 600 degree F.

The importance of cooking some foods, especially meats, is to kill bacteria.  Bacteria won’t grow below 41 deg F or survive above 140 deg F.  The internal temperature of meats needs to reach a range between 140 deg F and 165 deg F to be considered safe.  Seafood needs to be cooked to 145 deg F or hotter.  To rid poultry of salmonella, poultry must reach 165 deg F on the inside.  Egg dishes should reach the same temperature.  Trichinosis is halted by cooking pork to about 160 deg F.   Ground beef should reach 155 deg F for safety.

Solar stills

Back in the 1960’s a pair of PhD’s working in the soil hydrology laboratory for the USDA invented a solar evaporation still that could suck useful drinking water out of the ground.  Even in the arid desert around Tucson, Az. where they were located, they realized that the soil entrapped useful moisture.  Such a solar still is made by digging a pit in the ground, placing a collection pot in the bottom and covering the hole with a sheet of plastic.  Additional moisture could even be gathered by placing green vegetation under such a tarp.

It seems that the first evaporative solar stills were invented back in the 1870’s to create clean drinking water for a mining community as explained in an earlier post in this same blog named “The Nitrate Wars”.   This same distillation where moisture is evaporated before the condensation is collected, is employed in affordable, plastic-vinyl inflatable stills that can equip small boats and survival craft at sea.  Where once stranded fishermen and sailors faced a death by dehydration they now have the opportunity to create the drinking water they need from seawater.  Muddy or brackish germ infested groundwater can be reclaimed in the same way.

There are several possible techniques to employ and efficiency factors to consider when fabricating an evaporative solar still.  Obviously good direct sunlight is essential to their efficient functioning.  The ‘basin type” solar still is the most common type encountered and somewhat resembles a heat trap solar oven.  In a “tilted wick” solar still, moisture soaks into a coarse fabric like burlap and climbs the cloth before it eventually evaporates.  In higher latitudes ‘multiple tray’ tilted stills can be used, where the feed water cascades down a stairway of trays or shelves, allowing closer proximity to the glass and enabling steeper tilt angles for the panel to capture optimum sunlight.

Other liquids besides drinking water can be refined in an evaporative solar still.  Ethanol can and has been concentrated from mashes, worts, musts or washes using a solar still.   Since a distiller usually desires more direct control over temperatures however, he might consider solar stills to be practical only for so-called “stripping runs”.   Some of the earliest perfumes were created from fragrances collected by distillation.   Soaking wood, bark, roots, flowers, leaves or seeds of some plants in water before distilling the mixture, is a common way of obtaining aromatic compounds or essential oils.   Not all plant fragrances should be distilled but eucalyptus, lavender, orange blossoms, peppermint and roses commonly are.   The lightest fractions or volatiles of petroleum (like gasoline) separate at temperatures available in solar stills, but the heavier ones will not.  Theoretically it should be possible to place slip or crude oil into a solar still to separate out the gasoline and higher fractions.

Solar water & air heating

Most readers will have experienced how water trapped in a garden hose will get hot on a summer day.  Portable camp showers are simple black water bags, suspended at a little elevation and in direct sunlight to warm the water.

Where climatic conditions permit people may employ gravity fed or pump pressurized waterlines and tanks on rooftops or simply along the ground to achieve the same solar water heating effect.  Others may construct or install dedicated solar heating water panels to heat swimming pool water or to pre-heat water before it enters their home’s gas or electric water heating tank.

The construction of a solar water heater and a solar air heater can be very similar in concept.  Basically air or water is conducted through pipes or conduits to a panel where the heat exchange takes place.  Copper pipe might be the most desirable material to use in a solar water panel because of its pressure holding ability, resistance to corrosion and longevity.  Thin walled pipes of cheaper metals can be used to adequately exchange or transfer heat to air that passes through them.  A growing fad in the construction of homemade air-heating solar panels is to build the collector with empty aluminum beer or soda cans.  The tops and bottoms of the cans are punched or drilled out and the cans are glued together to form a continuous airtight pipes.  The box that holds everything is well insulated (sides and bottom) every interior surface exposed to sunlight is spray painted a dark, sunlight absorbing color – preferably using a high quality, high temp, UV protected paint.  A transparent glazing (of glass, plastic, fiberglass, Mylar, acrylic, polycarbonate, etc.) is tightly sealed over the top of the trap.  A double or even triple layer of glazing is preferable to a single one to reduce the escape of thermal heat.  While beer and soda cans are popular because of their availability and affordability, equally efficient collectors could be made from tin cans (made of metal called tinplate), rain gutter downspouts, old aluminum irrigation pipes, single walled stove pipes or even from bug screen like you’d find on a window.  This site, chosen from many that discuss solar heating with air, suggest that bug screen collectors are on par with soda can collectors and are possibly easier to construct.

In the choice of fan or blower used to push or pull air through the system, it is preferable to circulate a large volume of modestly heated air rather than a small quantity of thoroughly heated air.  Ideally a solar panel can increase the heat of the air passing through it as much as 50 or 60 degrees F.   In this type of collector an optimum airflow rate of 3 CFM per square foot of absorber has been suggested.  In general the larger the solar air panel, the better – small ones are probably not worth considering.  They should be built with quality paints, glazing and other components where possible to resist corrosion and decomposition from sunlight and other climatic elements.

Pointing solar panels

Direction

For optimum efficiency any solar panel should face the sun at a perpendicular angle.  The position of the sun changes constantly however throughout the day.  Some institutions or uber rich people might purchase solar trackers which employ servo or stepper motors to keep photovoltaic panels aligned with the sun.  Such ‘trackers’ increase overall efficiency by increasing morning and afternoon light collection.  The rest of us however have to make do with permanently fixed or periodically adjustable panel mounts.  Normally the bases of fixed panels are aligned perpendicular to due (not magnetic) south.  Some owners of grid tied solar photovoltaic panels however are deciding to aim their panels towards the west.

Tilt

The effectiveness or efficiency of a given solar panel is definitely affected by its proper orientation to the sun but as the sun moves around a lot, solar panels that do not automatically track its movement must seek a positional compromise.  The sun’s apparent altitude in the sky changes throughout the year.  Because of the earth’s motion the sun’s altitude appears to vacillate 23.5 degrees between summer and winter solstices or every 6 months.  Solar panels near the equator can be positioned parallel with the horizon and largely remain efficient by just pointing straight up.  The further a location is from the equator the more vertical a panel’s ideal tilt becomes.  Above the 45th parallel, vertically fixed solar panels mounted to the side of a building can preform admirably in the wintertime.  There is no one perfect tilt angle with which to keep a solar panel perpendicular with the sun’s rays throughout the year.  This fact motivates some people with adjustable panel mounts to periodically climb up on their rooftops with wrench in hand to refine panel tilt.  Others might wish to install a solar panel permanently in the best year round average position and not worry about adjustments.

Older literature for solar panel installation might quote a rule of thumb where 15 degrees are added to latitude for wintertime panel tilt, or 15 degrees of angle are subtracted from latitude to acquire summertime panel tilt.  A more modern set of calculations being mimicked or repeated often around the web, suggest wintertime tilts that are a bit steeper than common to capitalize on midday rather than whole day solar gathering and flatter than normal summertime tilts favoring better whole day rather than midday collection.

-To calculate the best angle or tilt for winter:

(Lat * 0.89) + 24º = ______   (The latitude is multiplied by .89 and added to 24 degrees)

-The best angle for spring and fall:
(Lat * 0.92) – 2.3º = ______

-The best angle for summer:
(Lat * 0.92) – 24.3º = _____

-The best average tilt for year round service:
(Lat * 0.76) + 3.1º = _____

For the purpose of illustration a latitude of 35 degrees North will be chosen.   Locations somewhat close to this latitude include: the Straight of Gibraltar, Tunis Tunisia, Beirut Lebanon, Tehran Iran, Kabul Afghanistan, Seoul Korea, Tokyo Japan – and in America, cities along Interstate 40 or the old Route 66 (Raleigh NC, Memphis Tennessee, Fort Smith AR, Oklahoma City OK, Albuquerque NM, Flagstaff AZ and Bakersfield CA).

 

A couple of sources for more information:

http://www.solar-facts-and-advice.com/

http://www.nrel.gov/rredc/solar_data.html