Viability of Green Energy Solutions

Green energy solutions work, on a limited scale. Solar, Wind, Biomass, Hydro-electric are all proven technologies. However, they each have limits to their use. Before committing to these technologies as the solution, we must understand their innate limitations.

Solar

In a sound bite, solar is energy from the sun. Limitless and free are the buzz words. However, how to capture this bounty is the catch. Currently there are two solar technologies that deliver this energy in a usable form, solar voltaic and solar thermal.

Solar Voltaic

Solar voltaic cells use the photoelectric effect to convert solar energy (light) into electricity. Current efficiency is less than 20% and usually closer to 10-15%. What that means is that for every 100 watts of power delivered to the cell, only 10-15 watts is recovered. Each solar panel has dozens of cells (Usually either 60 or 72) and usual capacity per panel is 350 watts for a 5.5x3.5 foot panel. The typical home uses between 7-20 kilowatts of power, doing the math that is 20-58 panels per home. Also required are inverters that convert the direct current (DC) power from the panels into alternating current (AC) used by modern homes. If you want to be independent from the power grid you also require a charge controller and batteries to store power for when the sun doesn’t shine. Usually, you need batteries for 3 days of no or low sun, so at an average of 13 KWx24hr*3 you need to support 936 KWh with your batteries. Most batteries are in Amp Hours so we need to convert between units, the needed amp hours to support 3 days without sun at 13KW per hour is 7,800 amp hours. Most batteries used in solar power systems are single units that you combine into banks, for 1148 amp hours of battery (a 12 battery bank) the cost runs about $25k dollars. I will leave the math for you.

Of course, the fewer modern appliances you use, the less electricity. Cut out your heat, AC, hot water heater, washer, dryer, dishwasher, TV and computers and game system and you can cut the amount needed considerably.

Large solar farms usually gather about 1 megawatt of power per 7 acres of land. The Sequoyah Nuclear station by itself generates 2,440 Megawatts of electricity on less than 600 acres. To replace it would require 4,880 acres of land which couldn’t be used for much of anything else.

Solar Thermal

Solar thermal uses the heat energy from the sun. There are three ways to utilize this heat, either use it to boil a fluid and drive a turbine, drive a heat engine such as a Sterling motor, or use thermionic conversion. The first two methods convert the heat into mechanical energy which is then used to drive a generator to produce electricity, usually this is about 30-50% efficient depending on how high a temperature the collector system can reach and the way the heat is converted to mechanical energy. Heat engines like sterling engines, are the most efficient but suffer scaling issues. Conversion of a working fluid to high pressure vapor and then use the pressurized vapor in a turbine is the preferred method as it also lends itself to backup using fossil or biomass fuels during no-sun periods. Many thermal plants store some of the heat gathered into molten salt or other heat storage methodologies for use during non-sun periods.

Most solar thermal plants are in high solar input locations such as deserts. Solar collectors can be either mirror or parabolic trough with the mirror type being more popular. The mirror type surrounds a central heat collector with solar tracking mirrors that reflect the solar energy to the central collector. For 100 MW it can require up to 5.5 square miles or about 34 acres per MW. Unless supplemented by battery banks, thermal storage or fossil fuel backup, these plants can only produce energy when the sun provides enough input, roughly 5-8 hours per day. To replace the Sequoyah Nuclear facility would require 82,960 acres of land.

Solar thermal accounts for thousands of bird deaths per year by birds getting fried by the intensified solar heat near the collector.

I didn’t discuss thermionic conversion because it is the least effective means of generation, at 5% efficiency, and is generally not used in commercial applications.

Wind

Wind power has been in use for centuries, filling ships sails and powering windmills that pumped water, ground grain, powered industrial processes and generated electricity. Modern wind turbines are 35-45% efficient at converting wind energy into electricity.

Each MW of electricity generated by wind requires 60 acres of land. To replace the Sequoyah Nuclear plant 146,400 acres of land would be required. The land around these behemoth generators, some reaching 500 feet in height from ground to blade tip, can be used, but no one wants to live in their shadow due to noise and the possibility of catastrophic failure during storms.

Currently there are approximately 15,000 abandoned commercial wind generators. Issues are generator maintenance, periodic blade replacement and disposal of the composite blades that are 116 feet long and weigh tons.

Wind generators only provide power when the wind reaches the critical generation speed. For most commercial wind generators, the critical windspeed is 11-14 mph. Obviously for average windspeed to be 14 mph for most of the time the wind will be at higher speed than this value for a good percentage of time. This limit on wind speed reduces the viability of wind generation as there are only a few places in the world where average windspeed is 14 mph. A quick internet search for a map of average windspeed at 30 meters in elevation shows that there are few places in the USA where wind generation is a viable technology.

For an example of wind turbine viability, I live at 2900 feet elevation outside of Hayesville, NC. I purchased a 1000-watt wind generator and placed it at 20-foot elevation. For the several months I monitored it I got no net generation of electricity, despite having winds at times more than 30 mph. As a point of comparison, the 1200-watt solar cell array I also used, powered my backup freezer with a 7 battery bank for non-solar viable days for the same period with no issues.

Biomass

As its name implies biomass generation is the burning or conversion of biomass to generate heat to power a steam turbine and use it to spin a generator. Biomass is leaves, woodchips, solid waste matter and possibly waste such as paper and cardboard. It is not carbon neutral but, if done correctly, generates less of a carbon footprint than fossil fuel-based generation. However, growing trees or other biomass simply to chop them up and burn them seems a bit retrograde and the shear amount of biomass needed to really make a dent in the hydrocarbon generation is prohibitive. Biomass is good for small generational requirements and maybe as a supplement for other green energy sources such as solar and wind.

Biomass is usually converted into energy by either direct burning or, by breaking the organic waste down using various chemical, heat or biological processes, into a combustible gas which is then utilized to generate electricity through normal steam plant or gas turbine generation.

Examples of biomass conversion is the heating of biomass without oxygen present to create a combustible gas, usually hydrogen, CO and CO2. Another is the use of microbes to process sewage or animal waste to produce methane which is then burned. One creative company uses wood gasification to power a combustion engine to drive a generator. During WWII wood gasifiers were used to power automobiles and other vehicles due to the shortage of gasoline. A recent example of vehicles using a gasifier was Eustice Conway on the show Mountain Men using one to power his work truck.

Hydro Power

Almost everyone has seen pictures of huge dams that use the water to power hydro turbines to turn electrical generators. No doubt you have also seen bucolic pictures of water wheels on old mills. Both are examples of hydro power. More esoteric examples also are available such as wave and tide power generation devices. For many years, before the electrification movement in 1936, many communities depended on small scale hydroelectric units for their electricity.

Hydro power is dependent on the movement of water, either vertical as in head, or horizontal as in flow. The amount of power available is directly relate to both. Head is the vertical distance the water falls. The use of dams to raise water levels is an example of head. The use of water wheels or submerged turbines in water currents is an example of flow.

Small scale, or micro-hydro generation is used in many remote regions to provide power for small cabins and workshops. As their name implies, they are usually in the range of 5kw to 100kw in capacity with many being far smaller. To use micro-hydro your site must meet flow and head requirements. For example, to match my 24KW solar installation, it would require at least 6-1/2 feet of head and a flowrate of 31,702 gallons/minute for a stream to produce the equivalent power.

One unique application of hydro power is the concept of a pumped storage facility. Power plants are more efficient when run at their rated power level, but electrical loads and vary considerably during the day. In a pumped storage facility excess capacity in the generators (such as is produced by a nuclear plant) is used to pump water to a reservoir which is higher than the source of water that is used to cool the plant (generally speaking) and then, when more power is required, the pumps become generators and the water is allowed to flow back into the source generating electricity as it does. An example is the pumped storage facility using in concert with the V.C. Summer Nuclear plant in South Carolina, or the ones used throughout the TVA generation area.

While hydro power is clean and non-polluting, the number of commercially viable sites is limited, and creation of dams can result in severe ecological ramifications. In addition, dams suffer from build up of silt which can limit their effective lifetimes.

Summary

We must utilize all sources of power, green or otherwise, but we must not do it blindly. Use of green solutions where it makes ecological and financial sense is imperative. However, to blindly use an inefficient or non-viable source of electricity because it makes us feel good is reckless and will result in damage instead of benefit.

We must pay attention to land use and restrict use of so-called green technologies when the long term cost could exceed short term benefits, land use surveys and long term ecological surveys must be done before any large scale use of solar or wind is attempted. The construction of dams is already limited by such restrictions. What works for a population of 5 million may not ecologically scale to a population of 300 million or more. We must use our resources and technology wisely.