How should I size my generating equipment? The answer is a resounding, “it all depends”. First of all it depends on the size and nature of the project.
- In prime power applications project sizing at 100-110% or more of the prior peak usage is recommended. If local utility prices are favorable, installing excess generating capacity may be attractive, if allowed under existing tariffs or a purchase power agreement (PPA) can be negotiated. Consideration should also be given to whether the project will include standby generators to support operations when the local utility grid is not available.
- If you are looking at co-generation , sizing will draw a balance between your thermal and electrical needs. Generally, but not always it is best to optimize around your thermal needs and let changes in your electoral load float on the utility grid. Here again, if local utility prices are favorable, installing excess generating capacity may be attractive. Similar considerations for standby generation should be considered here as well.
- For peaking / peak shaving applications, a wider degree of latitude for sizing can be considered anywhere from 0 to 100% or more, depending upon the needs and desires of the customer. Standby generation use of the equipment should also be considered when sizing.
- A good rule of thumb for standby generation is to install 110%-120 % of prior peak load.
Within reason, you are generally better to error on the side of excess capacity, to accommodate growth in the facility load over time, without the need to consider adding capacity in the coming years. Those incremental kw of installed capacity are relatively inexpensive compared to the cost of an additional generator and output breaker in the future. Further, the existing generation will run at a lower load, reducing the stress on the machines, especially during hot weather.
Depending on your need for reliability and your local utility tariff’s with regard to the costs for backup power should your prime power or co-generation plant trip off line during service you may want to consider:
- 3-to-Make-2 Strategy
Under this strategy, you divide your project sizing in half and then install three units. With this approach, you operate all there of the units at reduced capacity and if one of the units trip, the other two quickly ramp up to pick up the load that was dropped. Also, as maintenance, testing, inspection and overhaul windows eventually come due, you can shut down a unit without impacting your ability to supply power. The downside of this strategy is you will spend more in upfront capital on three small units rather than one larger unit.
- N+1 strategy.
In this strategy a number of smaller units are put down in blocks with N being the number of units needed to match the project sizing discussed previously and the +1 obviously meaning on additional unit. Like the 3-to-make-2 strategy all N+1 units would be operation sharing the load. While N+1 increases the capital outlay, it is a useful strategy, if the project would also involve standby generation because the N+1 configuration eliminates the need for redundant capacity.
If you are looking at prime power or co-generation applications, combustion turbines are often a better choice than reciprocating engines. Although there are some applications in which the efficiency of some of the higher end natural gas fueled reciprocating engines are making inroads into this space. If you are considering reciprocating engines for prime power or co-generation, a number of high-end manufacturers make slow speed engines suited for this purpose.
For peaking / peak shaving and standby generation, where starting reliability is at a premium and there will be far fewer operating hours, reciprocating engines are normally the better choice.