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.
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.
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 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 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.