Control Software and Renewable Energy, Part 1

By Béla Lipták, PE, Columnist

In this series of articles, I will give some examples of the control software that needs to be developed for renewable energy processes, and will compare it to the traditional industrial control systems. Traditional controls evolved from using single control loops. Renewable energy control is a step beyond that. Traditional industrial control concen-trated on keeping flows, temperatures, pressures, etc. at their desired values, so these had only an indirect influence on efficiency, productivity or profitability of the controlled unit operation. The main goal of renewable energy control will be to optimize efficiency and profitability, while treating the operating conditions only as limits of the operating envelope.

Traditional industrial control used multivariable control only in a few simple cases, while renewable energy control will always be working with multivariable control. In addition, while traditional industrial controls often separated the different states of control for start-up, normal, emergency and shutdown phases of operation, renewable energy controls will integrate these, and will reconfigure themselves automatically as they respond to market conditions, energy and raw material costs and other profitability-related changes.

To design any control system correctly, we must first understand its "personality" fully, both in terms of its operation and its gain and dynamic characteristics. Therefore, in this series, before giving specific examples of the software packages needed to automate such renewable energy processes, I will describe these processes. I will then describe the optimization controls for energy-free homes, reversible fuel cells and solar-hydrogen power plants.

Figure 1. Energy cycle without releasing carbon.

Later articles will describe the software needed to control the solar-hydrogen processes (Figure 1) 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 fuels 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, reversible fuel cells and "solar-hydrogen" power plants of the future.

The Renewable Energy Processes

During the industrial and post-industrial period (from the 18th to the end of the 21st centuries) we will have 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 2. 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 as nuclear and hydroelectric.

Figure 2. Past and future energy trends (blue = fossil, red = total, including nuclear and renewable energy sources).

We might also note 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 aging refineries. This is in spite of both nuclear and fossil fuels being exhaustible, and while both are getting more and more expensive.

The Biological Life Cycle

The biological life cycle on Earth 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 = 6 CO₂ + 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 imbalance 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, but without the use of carbon.

The yearly solar energy that is received on each square meter of the Sahara is approximately 3000 KWh. Approximately 2500 KWh/m2/yr is the "insolation" in southern California, and 1250 KWh/m²/yr in New York City or in Connecticut (where I live). I will use my house as an example of how an energy-free home could be designed, how its operation could be automated, and how the costs and payback periods can be calculated.

If my roof (450 m²) was covered by 10%-efficient photovoltaic (PV) solar collectors, assuming my wife allowed me to cut down the trees around our home, which she would not, the collectors would generate 54000 KWh/yr. Our yearly electricity consumption, including a pool, is 15000 KWh/yr, for which I pay about $3000. My yearly oil and gas consumption is equivalent to 864 gallons of oil, having an energy content of about 32000 KWh.

Today, in this area, the same energy in the form of oil costs about half as much as it costs in the form of electricity. (This is due to the low oil and gasoline taxes. In Europe and in other parts of the world, the cost of gasoline is about twice what it is here because of higher taxes.) Therefore, my yearly total energy use (expressed in KWh units) is 47000 KWh. This quantity is 7000 KWh/yr less than the amount of solar energy that can be collected on my roof. Therefore, this excess can be used to recharge a plug-in hybrid or electric car.

The installed cost of 10%-efficient shingles is about $500/m² or about $225,000 to cover my roof. In Connecticut, the government subsidy is 40%, lowering the total investment to $135,000 (without considering the added advantage of having new shingles). The local power company provides the bidirectional electric meter needed to connect to the grid free of charge.

The total value of 54000 KWh/yr of electricity (if purchased in the form of electricity at $0.2/KWh in our area today) is $10800. (If part 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 is, say, Nevada instead of Connecticut, the payback period is further reduced.