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Renewable Energy Processes, Part 2

July 2, 2010
The Control System Must Be able to Detect the Loss of the Grid and Respond by Isolating Itself from the Grid or by Tripping Offline
By Béla Lipták, PE, Columnist

The first article in this series described the bumpy energy road that mankind is traveling. It also showed that process control is key to smoothing this road even though it was totally neglected in the recent oil and mining accidents. Process control technology will come into its own as we develop the new, renwable energy processes.

The transition to this future—based on free, clean and inexhaustible fuels—will require not only an effort greater than the Marshall Plan, but also an concerted effort to improve the safety of the traditional operations today as we scrape the bottom of the barrel for the remaining fossil fuels. In the September issue, I will describe how process control can contribute to this effort.

In this series, I will focus on the control of renewable energy processes and the reasons why they require different approaches than do regular industry applications. I will also look at  the challanges of developing the required control software packages. In this article in the series, I will discuss the control of renewable energy use in private homes, starting with the simplest designs and gradually advancing to the more sophisticated ones.

Solar Hot Water Systems

If only hot water is to be generated, direct thermal solar collectors provide better efficiency than indirect photovoltaic ones. Figure 1 illustrates the simplest ("passive") solar hot water collector. Here, the hot water, because of its lower density, rises into the solar collector or the collector tank, and leaves it when a user valve is opened in the house. When this happens, the discharged solar hot water is replaced by fresh cold water. This system has no temperature controls, and therefore it can overheat. For this reason, a thermal expansion relief valve is usually added to protect the system from over pressure.

Figure 1 Passive solar hot water collector.

Figure 2 (top) illustrates the design of a "direct" solar hot water collector system controlled by a differential temperature switch (ΔTS). This switch starts a circulating pump when the discharge temperature from the solar collector is high enough.

Figure 2 (bottom) shows the operation of an "indirect" solar hot water collector system. This is similar to the direct one except that it has two circulating loops and a heat exchanger between them to transfer the heat from the antifreeze fluid circulating in one of the loops into the water circulating in the other. This configuration makes the system suitable for winter operation. Consequently, this system is safer and more convenient than the direct one, but is also less efficient and more expensive.

Figure 2. Temperature difference-controlled direct system (top) and "indirect" automatic solar hot water system.

Photovoltaic (PV) Systems

The photovoltaic (PV) solar cells generate DC electricity at 12 volts to 17 volts. This electric power can directly charge batteries or operate electrolyzers. If it is to serve the AC electricity needs of the home and the excess is to be sent to the grid, the generated DC electricity has to be converted into AC at higher voltages.In some installations, the generated electricity is needed in both DC and AC forms; therefore, combination systems are used.

PV Collectors and Their Control

Figure 3 describes the main components of a grid connected photovoltaic solar collector system, including the DC to AC inverter, fused DC and AC switches, the utility switch and the service panel, which can have two KWh meters. One of the KWh meters is tracking the total solar electricity production of the collectors while the other is bidirectional, tracking the electricity flow to and from the grid and totalizing the net electricity use or production of the household.

Figure 3: Components of a grid-connected PV system

Inverters are required to make the direct current produced by the PV cells compatible with the AC power distribution grid. These inverters are similar in function to the converters used in variable-speed drives. In order to provide a grid connected inverter, they must maintain the same frequency as the incoming grid.The control system must also be able to detect the loss of the grid, and respond to that by isolating itself from the grid or by tripping offline. The usual means of detecting a grid disconnect (or island) condition is to have the output frequency periodically drift up or, more usually, down; because if the grid is in operation, the phase shift will be readily detectable. This way, at two or three Hertz away from the nominal grid frequency, the inverter will trip to the island mode, and the system will either shut down (pending return of the main power supply) or disconnect itself from the grid.

An alternative method of detecting grid failure is to detect voltage deviation (either high or low) and set the inverter to trip if the voltage deviation moves outside a +10% and –6% band relative to the nominal supply values. This protects linesmen from possible backfeeds from generation systems, who might not be aware of this condition, when isolating the main supply. The system must detect a grid link failure within two minutes. Grid connected inverters can either drive into the grid as a power supply or can supply part of a load.

The third option is to drive the main's current draw to zero, which requires a more delicate balance. The choice among these options depends upon the specifics of the local electricity supply and upon the receptiveness of the supply authority to accept electric power from generators owned by others.

During daylight hours, the solar electricity generated might exceed the needs of the household and under such conditions, the excess can be sent to the grid or placed into local storage. The optimization software will consider the following factors in making this decision:

  1. Is there excess solar electricity available? If there is, is there room in local storage to receive more electricity? If the answer is yes to both of the above conditions, then ask the following questions:
  2. Is it more economical to send the excess to the grid during peak periods, if or when the rate at which the utility pays for the electricity is higher, or does it make more sense to send it to storage, as when the rates are lower?  
  3. If there is no excess solar electricity available and, therefore, in order to meet the needs of the household, additional electricity is required, should this additional electricity come from the grid, or can it come from the electricity storage of the home? The home storage optioncan only be considered if the storage devices (batteries or hydrogen) are full or are above minimum reserve needed for other purposes, such as transportation. If this is the case:
  4. Is it more economical to obtain the needed electricity from storage, which may be the case during peak periods if during these periods the charge by the utility is higher, or from the grid, which can be the case during the night if the cost of night time electricity cost is lower?

The safe, efficient and inexpensive storage and transportation of solar energy is the key to converting from today's exhaustible energy sources (fossil and nuclear) to inexhaustible, clean and free renewable energy sources. Therefore, I will discuss the control software needs of these new processes, but before discussing their control and optimization I will describe these processes themselves.

Solar energy can be stored in the forms of electric, chemical or heat energy. Storage in the form of electricity can use the grid or batteries. Storage in the chemical form can utilize several reactions, one of which is generating hydrogen fuel from water by electrolysis and storage as heat energy in hot water, hot oil or molten salt.

I will start this discussion in my next article by describing the state of the art of storing solar electricity in batteries.  

Béla Lipták, PE, control consultant, is also editor of the Instrument Engineers' Handbook and is seeking new co-authors for the for coming new edition of that multi-volume work. He can be reached at [email protected].

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