Solar Energy

 “[Nobel Laureate Wilhelm] Ostwald’s energetic imperative – Waste no energy but value it – is relevant as humankind makes the inevitable transition to a permanent economy based exclusively on solar radiation.” – Vaclav Smil

“My own preference is to fill the Mojave with solar concentrating plants, and save some of this wonderful stuff [oil] for our descendants.” Dave Rutledge

 “I’d put my money on solar energy… I hope we don’t have to wait til oil and coal run out before we tackle that.” – Thomas Edison

 When Edison and Ostwald were alive there were only 2 billion humans on the planet so capturing solar energy offered more than enough low entropy to support human society sustainably with a generous quality-of-life.  In 1972 when the world population was 3.8 billion, the authors of Limits to Growth concluded that global population and industrial activity were still below the levels that could be supported indefinitely by Earth.  Today there are 7 billion people and it is less clear that sustainability is achievable.  In the 2004 edition of Limits to Growth the authors concluded that both population and industrial activity had already grown above sustainable levels [Meadows, 2007].  Sometime in the not too distant future it may become impossible to achieve sustainability, if we do not act responsibly now.

 Fossil fuel resources are running out and we are only arguing about when.  As we will discuss in some future article, nuclear power is severely limited by the availability of uranium resources and reserves and it is a simple fact that we still have no idea how to process or store the waste nor how much whatever the solution (assuming there is one) will cost in terms of energy and money.  That leaves solar, which includes tidal, wind and biomass; and much more limited and dispersed geothermal energy. 

The radiation intensity of the sun through a plane perpendicular to the line connecting the center of the Earth to the center of the Sun measured at the top of the atmosphere is 1366 Watts per meter squared.  This is called the solar constant.  The Earth has a mean radius of 6371 kilometers.  The solar energy flux through the circular area projected by the Earth onto a planar surface is 1366 X 6,371,0002 X p = 174,000 terawatts.  

 According to the British Petroleum 2009 statistical review, humans consumed 11300 million tonnes of oil equivalent energy in the year 2008.  One tonne of oil can produce 12 million Watt-hours according to the same source.  There are 24 times 365 hours in a year.  Thus humans consumed energy at the rate of 15 terawatts in 2008.  BP excludes biomass, solar and wind but these numbers are accurate enough for our purposes. 

 Humans therefore consume energy at a rate equivalent to 0.008% of the sun’s energy flow.  We would have to intercept 6371 kilometers (km) squared times p times 0.00008 or 10,000 square km of land on Earth beneath the sun with 100% efficient solar collectors and while there is no cloud cover.  We can work out the other bits.  Solar panels are about 15% efficient and if we situate our collectors in the world’s deserts we can ignore any cloud cover.  We then have three remaining issues to consider.  The Earth is a sphere, it rotates and it has a 23.5 degree “seasonal” wobble.  Because the Earth is a sphere we only need 10,000 square km if we build our solar array at the point on the Earth surface directly below the sun.  If we are 10 degrees to the north then we only intercept cosine (10) of the total power or 98%, which is not bad.  But if we are 40 degrees to the North, we only intercept cosine (40) or 77% of the power.  Because the Earth rotates, our selected point 12 hours later will be in the dark of night.  And because the Earth wobbles once a year, the sun is directly overhead in the Sahara desert at noon in the summer but directly over Botswana during the winter. 

 To avoid cloud cover, the best places for our collectors are on the Tropic of Cancer, where the Sahara is located, or the Tropic of Capricorn, in the middle of the Australian Outback.  The cool thing to do right now would be to get a globe and play with it.  You will note that when the Earth rotates and positions the middle of the Pacific Ocean towards the Sun, there are no land masses available for our collectors.  So we would have to be able to store energy.  Concentrated solar thermal power promises to be cheaper than solar photovoltaic panels and easier to manufacture and heat is easier to store than electricity. The United States is a leader in this technology as we once were in photovoltaic technology as late as 1998 before we gave that lead up to Germany and China. 

Getting back to our 10,000 square kilometers, since collectors are 15% efficient, we need 67000 square kilometers and because our surface is not directly under the sun most of the time, we need to increase the area by another 40% or so to 90,000 and if we add a cushion for cloud cover lets round it up to 100,000.   This is a little less than the total area of Arizona. We need several such stations around the Earth to cover night and day.  Concentrated solar thermal power can work round the clock by storing heat generated during the day but it only collects the energy from solar photons during the day.

There are other things we can do though.  I have solar panels in my back yard here in Virginia.  They contribute.   Additional solar collectors can be distributed throughout the economy.  Integrating distributed and concentrated sources of energy is an important part of the Smart Grid evolution.  There are other ways to capture the sun’s energy such as wind, tidal and biomass.   Algae can be used as a solar collector [Westervel, 2010].  And finally, the biggest part of the solution to America’s energy problems, at least, is conservation.  We can simply use less. 

 Still, the big problem with this dream of sustainability is building out the necessary infrastructure.  The entire energy infrastructure we have now was build using fossil fuels when such energy sources had an energy return on energy investment ratio (EROEI) of about 100.  Because we are using all the easy stuff first, the light sweet crude and anthracite, what we have left is tar and lignite with EROEIs closer to 5 or less.  They are also dirtier in every respect.  We need to use the energy from remaining fossil fuels to construct our solar powered future.  Building more nuclear reactors has an opportunity cost since it wastes resources that could have been marshaled to build out solar plant and it has a huge liability cost since we do not know what we have to do with the waste and don’t know how much that will cost.

 Tony Noerpel


[Meadows, 2007] “Evaluating Past Forecasts”, in Sustainability or Collapse, eds. Costanza, Graumlich and Steffen, Dahlam Workshop Reports, 2007.

 [Westervel, 2010]


Passive Solar Design Overview: Part 1 – The Basics

Passive solar refers to the design and placement of a building to enable solar heating without the need for sensors, actuators, and pumps, in contrast to active solar, which utilizes pumps/blowers, sensors, and logic control units to manage collection, storage, and distribution of heat. The two techniques are not exclusive, however, and can work together effectively.

As solar radiation (insolation) is a diffuse energy source, and not at the beck and call of a thermostat, passive solar design techniques are at their best when combined with other related methods, such as energy efficiency (insulation, weatherization, building envelope minimization), daylighting, passive cooling, microclimate landscaping, and a conservation lifestyle (i.e., temperature settings, raising and lowering of insulated shades, etc). Most of these topics will be covered in other articles, though passive cooling will be addressed in this series, which is intended as an overview, as a complete engineering treatment on passive solar design would require several dozens of articles.

Even though solar insolation is diffuse, and generally weaker the further away from the equator, it can be the basis for the majority of a building’s heat energy input even in high latitude places such as Canada, Norway, Germany, the Northern US (Maine, New Hampshire, Michigan, Wisconsin, Minnesota, North Dakota, Montana, Idaho, Washington state, etc), Scotland, the Netherlands, etc. Even the US Department of Defense has a passive solar design guide. Design approaches such as Passivhaus have achieved up to 90% reduction in energy use over traditional building methods. In areas with reasonably consistent winter insolation, well insulated passive solar buildings with sufficient thermal mass storage can approach 100% of their space heating needs with passive solar. Enhancements can be added to existing buildings, through major or minor renovations, or through simple additions (Part 4 of the series).


The Greeks faced severe fuel shortages in fifth century BC, resorting to arranging their houses so that each could make maximum use of the sun’s warming rays. A standard house plan emerged, with Socrates noting, “In houses that look toward the south, the sun penetrates the portico in winter.” The great Greek playwright Aeschylus even proclaimed only primitives and barbarians “lacked knowledge of houses turned to face the winter sun”. The Romans picked up on this technique, and improved it by adding windows of mica or glass to better hold in the heat. They passed laws to protect the solar access rights of owners of solar homes from shading by new buildings. In the Americas, the Pueblo and Anazazi took advantage of solar insolation in their adobe and cave dwellings, respectively.

In the 18th and early 19th centuries, solar greenhouses became popular for those of means to grow exotic tropical plantlife in temperate climes. In the 20th century, German architects such as Hannes Meyer, director of the influential Bauhaus architectural school, urged the use of passive solar design techniques that began to flourish in the 1930s, only to be pushed aside by the Nazis and WWII. Many German architects made their way to the US, and a small solar market developed. Built in 1948, Rosemont elementary school in Tuscon obtained over 80% of its heat via solar means, but in 1958, with cheap energy now available and an extensive addition planned, the school district chose to go with a gas-fired furnace. The 1970s saw more emphasis on renewable energy, and passive solar became a household word, though still only penetrating a very tiny percentage of builders’ visions for the new homes market. More in-depth passive solar history details can be found at the California Solar Center.

The Basics

Location and Orientation

To assess whether passive solar is advantageous to a location, one must first find out the amount of winter sunlight that is available. The simplest way is to find solar insolation data for the site under consideration, ideally collected over a series of decades (noting that a changing climate can mean the data may need to be extrapolated). The data can come in tabular or map form, with the latter providing a quick indicator of the amount of winter insolation in one’s area. Tabular data, however, is more precise, giving one the best information available about trends in their area. A note of caution: the data is usually an average of conditions, and does not necessarily take into consideration unusual weather years or how the climate may change in one’s area of consideration.

Interpretation of Data:

Most of the maps and tabular data measure solar insolation as kWh/m2/day, which is roughly the number of kilowatt hours of energy striking a square meter of surface in a day. This is also referred to as a Sun Hours on some maps, and we will refer to it as such throughout this series. Important note: Since virtually all modern passive solar design focuses on vertical windows, data must be specified or converted to a vertical orientation. Some of the data currently available is for collectors tilted at an angle equal to the site’s latitude (L) or a horizontal surface (H), which would need to be converted to a vertical surface (V). The table below contains a partial list of solar maps and data, though make sure any source you use focuses on winter data, as other maps/data are used for year around solar photovoltaic projections.

Region Maps Data
World Solarex (L)
FirstLook (Americas only currently)(H)
WRDC (select Global)
Canada Solarex (L) WRDC (select Global)
Europe Satellite data map (H) WRDC (select Global)
US National Renewable Energy Labs
(select Vertical surface) (V)
Many US cities (H)
Detailed data (H) (manual)
– Other sources
Australia Aus BOM June Map (L) Aus BOM site data (L)

The orientation of the building will determine how much solar insolation is captured during the desired period of the day. For example, a passive solar house facing the equator will receive an equal amount of solar heat before and after noon. The more a building is oriented away from true south (or north in the southern hemisphere) the less winter solar insolation it will be able to capture, and it becomes more susceptible to undesirable summer solar energy that is harder to shade with a properly sized overhang.

In addition to direct solar insolation beaming from the sun, there is also diffuse radiation from the sky, and reflected radiation from the ground.

Figure 1 – Types of solar input

Design Aspects:

Passive solar building design revolves around 5 main aspects;

Aperature: The set of windows and overhangs that determine how much sun enters the building.
Absorber: The material that the sun’s ray come into contact with.

Thermal Mass: The material that stores the sun’s thermal energy for re-release after sundown.

Distribution: The means by which the thermal energy is released to the living/working spaces.
Control: The techniques used to control the collection and distribution of the sun’s thermal energy.

These aspects can be configured by the designer/architect into roughly three main design themes (with endless variations);

  1. Direct Gain: Sunlight shines into and warms the living space.
  2. Indirect Gain: Sunlight warms thermal storage, which then warms the living space.
  3. Isolated Gain: Sunlight warms another room (sunroom) and convection brings the warmed air into the living space.

Figure 2 – Direct solar gain

Figure 3 – Indirect solar gain

Figure 4 – Isolated Gain

In Part I of this series, we will cover the Aperature;


The first step in passive solar design is determining how to collect the sun’s energy. In most climates that passive solar is employed, this means windows of one form or another. An important metric of a window is its Solar Heat Gain Coefficient (SHGC) that measures how much of the sun’s energy passes through the window without reflection or absorbtion and re-radiation. The higher the SHGC, the more solar energy a window will allow through. A plain single pane window normally has a SGHC of 0.86 while a plain dual pane window is about 0.72.

In order to reduce heat loss in cool and cold climates, windows are normally at least dual pane, if not triple pane. The dead air space between window panes helps to increase the insulation factor, call the R-value (or its inverse, the U-value). One single pane of ordinary glass has an approximate R-Value of 0.85. A dual pane window with 3/8 inch of air space typically has an R-value of 2.1. The substitution of less viscous gases such as argon and krypton allow greater distances between panes before the gas begins to convect (tranferring heat at a much higher rate), increasing their insulating effect. Each pane added, however, blocks/absorbs/reflects more solar energy, which effectively reduces the window unit’s SHGC. Additionally, low-E coatings that help to reduce the amount of infrared heat radiated out of a room through a window also reduces SHGC (amount dependent on the type of low-E coating). So a balance must be struck by the designer/architect between the amount of energy received during winter sunlight hours vs. the amount of energy lost 24 hours a day. There are windows available that have been designed for passive solar applications to provide sufficient SGHC while still providing adequate insulation (e.g., one such window has a SHGC = 0.56 and an R-value = 5).

There is ongoing research to bring aerogel windows to commercial production, as these windows provide extremely high R-values (approximately R-10 per inch) while having SHGCs of .52 or greater.

The orientation of the window is just as important; windows facing the equator receive the greatest amount of sunlight. And this orientation also greatly reduces unwanted solar collection during the warmer days of the year, as windows facing East and West are difficult to shade effectively with simple overhangs, requiring larger and/or view blocking awnings.

Discuss this topic at the Sustainable Loudoun Forum under Energy: Passive Solar

Will Stewart


California Solar Center: Passive Solar History
US DoE Passive Solar Home Design