During the industrial and post industrial period (from the 18th to the end of the 21th centuries) we depended on exhaustible energy resources (fossil, nuclear, etc.), while by the beginning of the 22nd century, our energy sources will be inexhaustible ones. The present energy consumption trend (based on NASA data) is shown in Figure 1. This trend, up to 2009 (solid blue line) represents the actual global consumption of the exhaustible fossil energy sources used in units of quads, (Q = 1015 BTU). After 2009, the fossil fuel consumption trend (dotted blue line) shows, how our fossil energy supplies will get exhausted. The red line represents the total global energy consumption up to 2009 and includes the non-fossil sources, such a nuclear and hydraulic.
It might also be noted, that the consumption rate of fossil fuels has already exceeded their rate of discovery. Yet, as of today, our resources are still being spent on building new nuclear and fossil plants or on replacing our ageing refineries. This is in spite of both nuclear and fossil fuels being exhaustible and while both are getting more and more expensive. Canada is destroying her pristine nature with recovering oil from sand and the United States is already experiencing another Exxon Valdez accident by deep sea drilling, while knowing that our consumption is 25% and our total deposit is under 2% of the global.
Figure 1: Past and future energy trends (blue-fossil, red-total, including nuclear and renewable energy sources).
The biological life cycle
It is also important to understand the biological life cycle on Earth (fueled by solar energy), which is based on the balance and interdependence of animal and plant life on the planet.
Photosynthesis takes up half of this cycle. In this half, the vegetation absorbs carbon dioxide and using solar energy splits water into oxygen (which is released into the atmosphere) and hydrogen, which, – using a catalyst named chlorophyll -, combines with carbon from the atmosphere to produce food for animals and humans (Photosynthesis = H2O + Sun Energy + 6CO2 = C6H12O6 + 6O2). The other half of the biological life cycle is respiration in which animals and humans inhale the oxygen generated by plants and obtain their muscle energy by digesting (burning) the glucose, cellulose, etc. Produced by plants, while exhaling carbon dioxide (Respiration = C6H12O6 + 6O2 = 6CO2 + 6 H2O + energy).
When the half cycles of photosynthesis and respiration are in balance, the concentration of atmospheric CO2 is constant. This concentration was ~ 280 ppm for 500,000 years. Today it is 360 ppm and it is projected that by 2050 it will be over 500 ppm. This shows that plant and animal life on the planet is no longer in balance. This “unbalance” has passed the point, when it could be corrected by planting trees. In order to absorb the excess carbon dioxide generated by the burning of fossil fuels, we would need to plant forests on an area equaling the surface of another Earth.
The goal of renewable energy processes is to reestablish the balance of the photosynthesis and respiration processes. The solar-hydrogen processes can supplement the photosynthesis part (the role of plant life), but without the use of carbon.
This series of articles will describe the software needed to control the solar-hydrogen processes (Figure 2) that form a cycle by substituting photosynthesis with photo-electrolysis (H2O + Sun Energy = Stored H2 + Released O2) and respiration with fuel cell (H2 + O2 = Electric energy + H2O) processes. This way, the increasing energy consumption of mankind can be met without releasing any carbon into the atmosphere and without the use of exhaustible energy resources, such as fossil or uranium.
Following this general introduction, I will describe the control software needs of three solar-hydrogen processes, which can fully automate the operation of the “energy free” homes, the “reversible fuel cells” and the “solar-hydrogen” power plants of the future.
Figure 2: Energy cycle without releasing carbon
The Energy Free Homes
The yearly solar energy received on each square meter of the Sahara is approximately 3,000 KWh. Approximately 2,500 KWh/m2/yr is the „insolation” in southern California and 1,250 KWh/m2/yr in New York City or in Connecticut (where I live). I will use our own house as an example to first show how the costs and payback periods of an energy free home installation could be calculated and later I will describe its design and optimization.
If our roof (450 m2) was covered by 10% efficient photovoltaic (PV) solar collector shingles (Figure 3), – assuming that my wife allowed me to cut down the trees around our home, which she does not -, these solar collectors would generate about 54,000 KWh/yr. Our yearly electricity consumption, including that of the pool pump is 15,000 KWh/yr, for which we pay slightly less than about $3000. Our yearly oil and propane consumption is equivalent to 864 gallons of oil, having an energy content of about 32,000 KWh.
Today, – in our area – energy in the form of oil costs about half as much as it does in the form of electricity, because our oil taxes are rather low. (Elsewhere in the world, it is about twice.) Therefore, the yearly total of our energy use (expressed in KWh units) is 47,000 KWh. This quantity being 7,000 KWh/yr less that the amount of solar energy we can collect the excess can be used to recharge a plug-in hybrid or electric car.
The installed cost of the solar shingles is about $500/m2 or about $225,000 to cover the roof. In Connecticut the government subsidy is 40% lowering the total investment to $135,000 (without considering the added advantage of having new shingles on the roof). The local power company provides the bidirectional electric meter needed to connect our electricity generation to the grid free of charge.
The total value of 54,000 KWh/yr of electricity, if purchased in the form of electricity at $0.2/KWh is $10,800. If some of it is purchased in the form of fossil fuels it is less, but that cost is also rising. Therefore, if we base the calculation on the present cost of electricity, the payback period is 14.5 years. Naturally, if electricity costs rise or if collector costs drop and efficiencies increase, the payback period will be shorter. Also if we deduct from the total investment the value of covering the roof with new shingles, or if the location of our home was say Nevada instead of Connecticut, the payback period would be further reduced.
Figure 3: Installing solar shingles, which can be used instead of regular ones or added on top of existing ones.
The Means of Storing Solar Energy
The safe, efficient and inexpensive storage and transportation of solar energy is the key to converting from today’s exhaustible energy sources (fossil and nuclear) to inexhaustible, clean and free renewable energy sources. Therefore, I will discuss the control software needs of these new processes, but before discussing their control and optimization I will describe these processes themselves.
Solar energy can be stored in the forms of electric, chemical or heat energy. Storage in the form of electricity can use the grid or batteries, storage in the chemical form can utilize several reactions one of which is generating hydrogen fuel from water by electrolysis and storage as heat energy in hot water, hot oil or molten salt. I will start this discussion by describing the state of the art of storing solar electricity in batteries.
Storing Solar Electricity in Batteries
Solar electricity can be stored in separate battery blocks (Figure 4) or in the batteries of electric or regular cars. The stored energy can provide nighttime electricity for the home, can charge the car batteries and can be sent to the grid. The time when excess solar electricity is available usually coincides with the times when the air conditioning loads are high and therefore this electricity is helpful to the power company as it reduces the peak load on the grid. One of the tasks of the optimization software is to direct the excess electricity to meet all needs while maximizing profitability considering the differences in the values of electricity at the various destinations.
Figure 4: The main component of a grid connected home that is provided with battery storage for the excess electricity generated by the PV cells.
The performance of batteries has improved during the last decades. The energy density (Wh kg−1 ) of lead-acid batteries used to be about 50 Wh kg−1 and their life expectancy was between 2 to 5 years. For example a 6 Volt, 210 Ah (ampere hour) golf cart battery stored about 1.26 KWh (Volts x Ah = KWh, 6 x 210 = 1.26 KWh) and weighed about 30 kilograms. Trailers or motor homes usually use only a single 12-volt house battery. These batteries often use Gel Cell or Absorbed Glass Mat (AGM) designs instead of Wet Cells, because these are suited for harsher environments, require less maintenance and provide the greatest reserve capacity (Ah = ampere hour).
The individual cells of lead-acid house batteries generate about 0.8 Volts and two 12 Volt batteries can power 4 KW inverters. Table 5 gives the size codes commonly used for batteries.
Table 5 Common battery size codes (ratings are approximate)
Table 5 Common battery size codes (ratings are approximate)
|U1||34 to 40 Amp hours||12 volts|
|Group 24||70-85 Amp hours||12 volts|
|Group 27||85-105 Amp hours||12 volts|
|Group 31||95-125 Amp hours||12 volts|
|4-D||180-215 Amp hours||12 volts|
|8-D||225-255 Amp hours||12 volts|
|L-16, L16HC etc.||340 to 415 Amp hours||6 volts|
Lithium ion batteries improved on the energy density of batteries to about 70 Wh kg−1. The energy density in the lithium-ion batteries that are used in electric cars is over 200 Wh kg−1, their power density is over 300 W kg−1 and their volumetric energy density is up to over 500 Wh l−1. The lithium-ion battery life has not improved much (2 to 5 years), but the newer NiMH batteries do last the life of the car and their charge to energy efficiency has reached nearly 90%.
Both the driving range and economics of batteries are improving with time. An electric car, – depending on its size -, requires between 0.25 to 0.5 KWh electric energy per mile of driving and the cost of today’s batteries is about $500 per KWh. Therefore, the purchase price of a battery block large enough for 1OO miles of driving between refills or replacements is $12,50 to $25,000. It is an open question, if the electric car “filling stations ” of the future will just replace the empty battery blocks with full ones, which would take a couple of minutes or will be operating on a “plug in recharge” basis, which takes longer. In any case, today the high battery costs still limit the use of all-electric cars.
On the other hand, the battery cost component in today’s hybrid cars is much lower. For example, the battery block in a Prius hybrid car costs about $3,000. It is projected that by 2012 the battery costs will drop to less than half of today’s cost (to about $200/KWh) and will continue to drop further as mass production starts.
As to operating costs, the economics already favor the electric car over the internal combustion (IC) engine. At a cost of $3/gallon and at an average fleet mileage of 30 mpg, the cost of driving an IC engine car is about $0.1 per mile. As the energy content of a gallon of gasoline is about 10 KWh, at a unit cost of $0.15 per KWh and with an electricity consumption of 0.3 KWh per mile, the cost of driving an electric car is about half of the IC type (0.15 x 0.3 = $0.045). Naturally, as the cost of gasoline rises and as the cost of batteries drop, the economic advantage of driving electric cars will further increase.
➢ Nissan Leaf goes on sale in 2010 for around $21K
Today, India’s Revai is the world’s most popular electric car
Figure 6: Electric Cars
The race between fuel cells (FC) and batteries is not yet over. Today, economics favor the batteries, but the long-range outcome in undecided. As electric cars take over as the means of transportation the availability and cost of the materials used will become an important factor. For the fuel cells it will depend on the cost and availability of the catalysts and for the batteries it will be the cost and availability of lithium, nickel, etc. As will be discussed later, in the paragraph dealing with reversible fuel cells (RFC), extensive research efforts are in progress to develop inexpensive catalysts and to increase efficiency by exploiting nanotechnology.
The writes is author of “Post Oil Energy Technology”
And Editor of the “Environmental Engineers’ Handbook”