Contents

7.1 Control Techniques

7.1.1 Threshold Control

7.1.2 Buyback Priority

7.1.2.1 Simple Buyback

7.1.2.2 Net Metering Control

7.1.3 Cooling/Heating Priority Control

7.1.4 Optimal Control

7.1.5 Complete Optimization

7.2 System Modeling

7.3 Examples of Control Operation

7.3.1 Sit-Down Restaurant in San Francisco

7.3.2 Supermarket in Chicago

7.3.3 Medium-Sized Office in New York City

7.4 Virtual Generation Plant (VGP) Control 7.4.1 Basic Goals for VGP Control

7.4.1.1 Minimization of Required Data Transfer between DG Units and Central Site

7.4.1.2 Reliability

7.4.1.3 Conjunctive Billing

7.4.1.4 Selection of DG Priorities

7.4.1.5 Buyback and Retail Wheeling

7.4.1.6 Cost Function Minimization

7.4.1.7 Arbitrary Load Shapes

References

This chapter summarizes some of the methods used to control distributed generation (DG) at the local building or campus level. This problem is important because the control mode can make the difference between a profitable

DG installation and one that is not profitable, all else being equal. Distributed power offers many different services. For example, combustion-based DG can provide significant amounts of heat to a building's space and water heating loads if cogeneration is used. When coupled with absorption cooling, the available heat can also be used to supplement the building's conventional cooling system (Kreider and Curtiss, 2000). The trade-off, of course, is that the cost of gas consumption increases while the cost of grid electricity decreases. The optimum control of such systems is the subject of this chapter. Optimal control maximizes the financial return on DG system investment.

This chapter does not discuss internal controls provided by manufacturers of DG equipment. Safety items, combustion control, alarms, and many other features of control systems are the province of hardware providers. This chapter addresses the best way to control DG systems that have properly engineered local control systems that ensure nominal operation of DG hardware. Given that, the principles here can be used to maximize revenue to the DG system owner. Figure 7.1 is a schematic diagram of a combustion-based generator. It is assumed that this generator is capable of providing heat recovery to a building.

Controller

Controller

FIGURE 7.1

Components of a typical distributed generation system.

FIGURE 7.1

Components of a typical distributed generation system.

7.1 Control Techniques

The control techniques chosen for DG will depend on the type of equipment installed. In the case of wind or solar power generation, the main goal is to produce as much energy from the system as possible to recover the installation cost. For combustion-based processes, however, the costs of fuel and maintenance must also be taken into account. The fundamental goal of the control scheme is to determine whether or not the on-site generation should be operating during a particular hour.* Generally, a simple hour-ahead

* Here the time internal is taken to be an hour, but any period can be used. For example, many demand rates use 15-minute periods.

control method is sufficient if the start-up transients of the generator are short. This section, adapted from Curtiss (2000), describes some of the techniques that can be used to control on-site generation.

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