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The global energy future – Part 3

ControlGlobal.com

Keywords: global warming, solar and energy

Control's own Béla Lipták continues his series on the future of global energy, stating that in order to obtain maximum energy recovery, the various solar collectors described all need to track the sun.

By Béla Lipták, PE, CONTROL Columnist

In the long range, there is no such thing as clean fossil or safe nuclear energy, and both will run out anyway. Uranium will run out even sooner than will the fossil fuel deposits. In addition, all operating nuclear reactors have analog controls, so if digitally controlled new plants are built, they will be subject to Murphy’s Law. As to fusion reactors, they operate at millions of degrees of temperature, so forget about them on earth. Use the safe energy source—the sun.

My books and articles in the past concentrated on the optimization of existing processes. In order to obtain maximum energy recovery, the various solar collectors described in this series of articles all need to track the sun. The best control algorithm to use is the envelope or herding strategy, which I described in my “Lessons Learned” columns in the January 1998 issue ("Buildings, Bureaucrats, and Brutes,") and November 1998 issue ("Envelope Optimization") of CONTROL magazine.

The parabolic photovoltaic (PV) collectors combine the steam generation capability of the thermal collectors with direct electricity generation by PV cells. In Figure 1 below, the silicon solar cells bonded to the coolant tube generate electricity, while the high-temperature coolant generates steam.

FIGURE 1: COMBINES SOLAR COLLECTOR

In this parabola-shaped trough, silicon solar cells are attached to the coolant tube at the focal line, which generate electricity, while the circulating coolant inside the tube provides heat or steam.

Solar Updraft Tower
In a number of previous articles, I have described the control systems for chimney-effect processes. (See Chapter 8.2, Vol. 2, Instrument Engineers Handbook, [IEH] 4th ed.) In high-rise buildings in the winter, this effect increases the heating load, because at the ground floor, cold air is pulled in by the chimney effect and has to be heated. I have eliminated this effect in the IBM headquarters building in New York by equalizing the inside and outside pressures at the ground floor. The same kinds of controls are applicable to solar updraft towers.

The chimney effect generates an updraft because the  heavier cold ambient air enters at the bottom of the tower and pushes the lighter warm air up the chimney. This upward air flow is caused by the pressure difference between the cold air on the outside and the warm air on the inside. Static home cooling systems use this effect to pull the cold air into self-cooled homes from underground ducts.

The solar updraft tower (see Figure 2 below) is an energy converter that converts solar-based thermal energy into concentrated aerodynamic energy (wind). In this system, air is heated under a circular greenhouse-like canopy. The roof of this canopy slopes upwards from the perimeter toward the center, where the tower stands. Under this canopy, the sun heats the air, which rises up the tower and generates electricity by driving an array of turbine generators.

FIGURE 2: THE SOLAR UPDRAFT TOWER

The solar updraft tower is an energy converter that converts solar-generated thermal energy into wind energy.

This “low-tech” solar energy collector concept is over a hundred years old, but the first 50 kW working model was built only in 1982.

The chimney of this model had a 10-meter (33') diameter and was 195 meters (640') tall, while the diameter of the canopy was 244 meters (800'), about 11 acres or 46,000 m2. This prototype operated for nine years and reached a maximum production of 50 kW.

Storing and Transporting Solar Energy
The storage of solar energy is an important consideration, because storage is required to compensate for the diurnal, seasonal and weather-related variations in insolation (the amount of solar energy received on a unit area). Therefore, in order to supply the continuous energy users without interruptions, the generated electricity must be stored. On small installations, hot water tanks or high-density batteries can provide storage. On mid-sized installations, pumped hydro storage can be considered. For larger installations, the compressing of air into underground caverns has been suggested. (see Chapters 8.15 & 8.41, Vol. 2 IEH, 4th ed.).

A better option is to eliminate the need for storage. This can be achieved if an electric grid is available in the area, and the utility serving the area is required to take the excess solar electricity and supplement it when more is needed. In this case, if the solar-power plant is located close to a hydroelectric or fossil power plant, it is possible to increase or decrease the fossil or hydraulic power plant’s rate of generation as the availability of solar energy changes.

Storing Solar Energy as Chemical Energy
A favored method of storage is to convert solar energy into chemical energy (convert it into a fuel) and store/distribute it in that form. The carriers of this chemical energy can be gases, liquids or solids.