Control Software for Renewable Energy - 6

In the past, humans struggled to conquer physical nature, while today they must conquer their own natures. Both history and literature tell us this won't be easy. What is new in the struggle is that now we have come to the realization that we are destroying our own environment and exhausting our resources. In 2010, global energy consumption was about 0.5 ZJ (zettajoule = 1021 joules). Eighty-two percent came from fossil, 6% nuclear, 7% hydraulic, 6%  biomass, <1% renewable sources. By the end of the century, this consumption is expected to reach 1.0 ZJ. The total proved exhaustible energy deposits today amount to 40 ZJ (coal ~ 20, gas ~ 9, oil ~ 8, uranium ~ 3), and the rate at which energy consumption is rising already exceeds the rate at which new deposits are being discovered.

The yearly renewable energy available is about 5,600 ZJ/yr (solar ~ 5,500, ocean currents, wind ~ 80, geo/ocean thermal ~ 32, river/tide/wave ~ 8, biomass ~ 4). Therefore, the continued dependence on the remaining exhaustible deposits (40 ZJ) is shortsighted and is likely to have serious geopolitcal consequences.

[pullquote]New ideas always encountered intense opposition. ("And yet it moves!" said Galileo Galilei.) The opposition that Galilei faced and the opposition faced by those who want a nuclear-free world or conversion to renewable energy is not much different. The vested interests in the current energy paradigms are powerful. Understandably enough, they are reluctant to let go of their current profits. They argue that change is not urgent, that the new ideas will not work, or that they are too expensive. But the fact remains that the current paradigm is not sustainable. And there are alternatives.

The Permanent Solution

The mothers of life on Earth are the sun and water. Therefore, in order to reach a clean and inexhaustible energy future, we should stop making energy by splitting uranium or oxidizing carbon and should obtain all our energy from the sun, distribute it (wirelessly) in the form of hydrogen, which at the point of use is converted back into electricity by oxidizing the hydrogen into water.

In the previous articles, I have described the technology to transport the solar energy from the Sahara or from the Mojave Desert to run our industries, homes and transportation. I described the controls needed to operate energy free homes with roofs covered by solar shingles, the electric cars using battery-swap capability, and the hydrogen burning power plants.

In many parts of the world, there is no electric grid or the electric companies do not provide two-directional electric meters. Under these conditions, self-supporting solar packages are needed. Therefore, in the next paragraphs I will describe the key component of a wireless and fully distributed solar-hydrogen system, the reversible fuel cell (RFC).

The Reversible Fuel Cell

One of the key components needed to convert to a renewable energy technology is the RFC. This device can be visualized as a two-directional fuel cell. The RFC does not exist today, but in the wireless and fully distributed energy economy of the 22nd century, it is likely to be as common as PCs are today. One of the key requirements of developing a safe, efficient and inexpensive RFC is good process control.

In a reversible fuel cell, the functions of the electrolyzer and the fuel cell are combined into a single unit, which in one direction (when solar energy is in excess) can operate as an electrolyzer, and in the other direction (when solar energy is insufficient or unavailable) as a fuel cell. Therefore, at night or on cloudy days the RFC is automatically switched into its fuel cell mode to generate electricity from the stored hydrogen. The automation of this operation is described in detail in my book Post-Oil Energy Technology.

It takes the same amount of energy to split water into hydrogen and oxygen as the energy that is obtained when hydrogen is oxidized into water. The only difference is that electrolysis increases the entropy, and therefore, not all the energy needs to be supplied in the form of solar electricity because some of the energy is supplied as thermal energy by the environment. Inversely, when the RFC is operated in the fuel-cell mode, part of the energy in the hydrogen fuel is released as heat. This waste heat can be used to heat water, air condition buildings, etc. Therefore, the electrolysis mode of operation (blue lines in Figure 1) requires heat, and the fuel cell mode (red lines in Figure 1) releases heat.

Figure 1: The reversible fuel cell (RFC) will probably use the proton electrolyte membrane (PEM) design. The blue lines show the fuel cell mode of operation, while the red ones show the electrolyzer mode.

Controlling the RFC

Once the RFCs are ready for marketing, they must be as simple to operate as are today's thermostats. The control software of the RFC will integrate four software packages. One will control the system in the electrolyzer mode. The second sub-package will control the RFC in the fuel cell mode. The third sub-package will control the automated steps of conversion between the electrolyzer and the fuel cell modes. The fourth sub-package will interface with the users of electricity in the household, the insolation detectors and past demand data in order to optimize the total system.

The role of process control is critical to the safe operation of the RFC. The complexity of the control challenge can be appreciated by realizing that operating a 400-cell RFC stack is like operating 400 pumps in parallel. Similarly, the switching between operating modes is like automatically terminating the operation of one process, purging the equipment to get it ready for the operation of a different process which has flows in the opposite direction, and starting that process. Fortunately, the switchover from one mode to the other does not need to be done quickly and is predictable.

The material and heat balance controls in both of these modes require accurate and high-rangeability flow detectors and a massive quantity of other, highly sensitive sensors. Sensitivity and reliability are both required because the cell diaphragms, for example, must be protected from high pressure differentials, and leaks must be immediately detected for safe and automatic shutdown. All the sensors and control chips will have to be miniaturized, accurate and inexpensive, just like the ones used in today's automobile industry where we have some 500 sensors in one car.

Conclusions

It is debatable how much fossil or nuclear fuel resources remain and how much climate change we can live with or how long we can use the oceans, ground water and atmosphere as garbage dumps. What is not debatable is that the conversion to a clean, free and inexhaustible energy economy is unavoidable, and the sooner we start that conversion the smoother the transition will be.

We should begin designing prototype equipment and operating fully automated pilot and demonstration plants to prove feasibility, safety and affordability. The process control profession will meet its share of the challenges of this transformation and will play not only a key role in this third industrial revolution, but also will gain the respect it deserves as one of the most important fields of engineering.

Béla Lipták, PE, control consultant, is editor of the Instrument Engineers' Handbook. He can be reached at [email protected].