Control Software and Renewable Energy, Part 3

By Béla Lipták, PE, Columnist

Process control is the key to both the efficiency and safety in all industries. This is particularly true in case of newer industries, such as renewable energy or deep-sea or low-temperature oil drilling. In this series I am discussing the control of the renewable energy precesses that use energy sources like the sun (both directly and as hydraulic and wind energies), from the moon (tides), from the rotation of the earth and its Coriolis effect (ocean currents) and from the heat below the ground (geothermal). Many of these industries are still manually operated. Some newer energy processes, such as offshore drilling for oil, are unsafe and inefficient because their designers are ignorant of process control.

Let me mention just one of many examples of the cost of this lack of knowledge: The laying of pipelines on the bottom of the ocean is still done by separately and manually controlling the speed of pipe production and the speed of the ship. Obviously, whenever the two speeds differ, the pipes break, causing large losses in this $1 million-per-day process. Yet, because the designers are ignorant of process control, they did not close the loop by detecting the tension in the pipe (the controlled variable) and keeping this tension constant by manipulating the speed of the ship. Instead, they just accept the losses as unavoidable, accept that human operators are imperfect and yet continue operation in the manual mode, while spending a fortune on underwater welding and other similar tasks.

The same holds true in many other new processes. In this series, I am discussing one process at a time. Below is the third installment leading up to the software requirements of solar-hydrogen power plants, their control and optimization.

The Means of Storing Solar Energy

The safe, efficient and inexpensive storage and transportation of solar energy is the key to converting from depending on today's exhaustible energy sources (fossil and nuclear) to inexhaustible and free renewable energy sources. As we know, solar energy can be stored in the forms of electric, chemical or heat energy. Storing this energy in the form of electricity can use the grid or batteries; storage in the chemical form can use the generation of hydrogen fuel from water by electrolysis, and storage as heat energy can use 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 car batteries or in storage battery blocks (Figure 1). The stored energy can provide night-time electricity for the home, can recharge electric cars 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 ("peak electricity"). This electricity is valuable to the power companies because it reduces the peak load their plants have to meet at a time when the plants are operating near their full generating capacity. In operating such a process, the task of the optimization software is to send the generated electricity to the most profitable destination.

The grid-connected home

Figure 1: The main component of a grid connected home that is provided with battery storage for storing the excess electricity generated by photovoltaic (PV) solar cells.

Trends in Battery Designs

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 two and five years. For example a 6-volt, 210 Ah (ampere-hour) golf cart battery stored about 1.26 KWh (V 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 per cell and two 12-volt batteries are needed to power 4-kW inverters. Table 1 lists the size codes commonly used in the grouping of batteries.

Table 1: Common battery size codes (ratings are approximate)The introduction of lithium-ion batteries improved the energy density of batteries to about 70 Wh kg⁻¹. The energy density in the lithium-ion batteries used in electric cars is higher (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 of two to five years has not improved much, but the life expectancy of the newer NiMH batteries is much better. They last the life of the car and their charge toenergy 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 100 miles of driving between refills is about $12,500. It is an open question if the electric car "filling stations " of the future will be designed to just replace the used 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's the high battery costs still limit the widespread use of all-electric cars.

The battery cost component in today's hybrid cars is much lower. For example, the battery block in a Prius hybrid car costs about $3000. 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 increases and new designs emerge.

While the first cost is still higher, the operating costs already favor the electric car over the ones using internal combustion (IC) engines. At a gasoline cost of $3/gallon and at an average fleet mileage of 30 mpg, the cost of driving a conventional car is about $0.1 per mile. The energy content of a gallon of gasoline is about 10 kWh; therefore, the per mile energy consumption of the average IC vehicle is about 0.33 kWh/mile (10 kWhpg/30 mpg = 0.33 kWh/m). The cost of driving an electric car is about half of that, because at an electricity cost of $0.15 per kWh the cost of the energy needed per mile energy (0.33 kWh/mile) is $0.045 (0.15 x 0.3 = $0.045) instead of $0.1 for the IC engine driven one. Naturally, as the cost of gasoline rises and as the cost of batteries drop, the economic advantage of driving electric cars will further increase.

Figure 3: Electric Cars

The race between fuel cells (FC) and batteries is not yet over. Today, economics favor the batteries, but the long-range outcome is yet undecided. As electric cars replace today's vehicles, the availability and cost of the materials needed for fuel cells and batteries will become important factors. The critical cost factor in the case of fuel cells is the cost and availability of the catalysts, and the critical cost for the batteries is the cost and availability of lithium, although other materials (nickel, manganese, antimony and the use of multiple miniature carbon terminals) and other designs (asymmetric super-capacitors, MEMS, etc.) are also being considered.

As will be discussed later, extensive research efforts are in progress to develop inexpensive catalysts and to increase the efficiency of fuel cells by exploiting nanotechnology.

Solar Battery Charge Controllers

The larger the solar electricity collection system, the more sophisticated the battery charge controllers become. They usually are provided with digital interfaces and interactive displays, which can be integral with the controller or can be provided with wireless connection to the user's PC. In either case, the human-machine interface allows the home's owner to check the charging voltage and the amount of electrical energy stored at any time and to modify the limit settings for each mode of operation. Some of the charge controller suppliers include Xantrex, Morningstar. Outback Power, Blue Sky Energy and Steca.

For very small systems, a 30-ampere controller with LCD display costs about $50, while the solar inverter needed can be as inexpensive as $10. Even these least expensive voltage controllers are usually provided with such features as: