The electricity grid delivers different amounts of power at different times, depending on the demand. Generally daytime has higher demand than nighttime because people are awake and using their electric devices. Generally midsummer is a high demand time in warm, wealthy countries because people will use a lot of electricity on air conditioning units at this time. In cold climates, mid-winter is high demand for many reasons, including lights, warmth, and car block heaters.
Summer or Winter Peak
The ‘peak’ energy usage refers to the most energy used in a certain interval of time. For example, if we look at an entire year in Saskatchewan, Canada, we will notice that the most electricity demanded at once is in the coldest part of the year. We refer to this as a winter peak. In other places, even within the same country, we can see instead a summer peak. In Ontario, they actually demand more electricity in midsummer than midwinter. This is primarily due to the electric demands of air conditioning systems, which have become increasingly popular in the developed world in the last several decades.
It is important to keep in mind that quite often the greatest energy demand during a year will be on the hottest days or the coldest days. Each presents a different technological challenge to those who design and operate a power grid.
In the most general sense, we are talking about moving power from one place to another. The electric grid accomplishes this by having power lines between generation stations and demand locations such as homes and businesses. Some general rules apply to this sort of technology. The more power you have to move, the more expensive it will be to build the infrastructure to do it. The further you have to move the power, the more energy losses you are going to have in doing so. These rules apply in general, but the specifics of a problem will dictate what sort of solution is applied.
For instance, sometimes long-distance, high-power transmission can be the best answer if an excellent power source just happens to be far away from demand. In such a situation, the costs of the transmission infrastructure are factored into the project from day one to determine the real feasibility of the scheme. A good example of development of this type is the James Bay Project in Quebec, Canada. This project built an astonishingly large hydroelectric facility several hundred kilometers from the major cities of the region such as Montréal. The project was well-conceived however, and has proven to be a good investment despite these challenges.
Capacity factor refers to the amount of power that a power plant produced compared to the amount that it could nominally have produced if it were running at maximum the entire time. For example, lets say that a nuclear plant that could provide 1 GW of power normally was shut down for one month out of ten. So for nine months it produces power at 1 GW, and for one month it produces nothing as it is being refueled or goes through a maintenance cycle. For this ten month period it would have a capacity factor of 90%.
Another example, this time with wind power. The Centennial Wind Farm in Saskatchewan in its first year of operation had a 42.4% capacity factor1. The farm is 150 MW, so we could say that on average it was producing about 42.4% of that, or 63.6 MW. This may not sound very good, but this is actually a very impressive capacity factor for wind power compared to most other places. Many nations with tremendous wind investment have average capacity factors in the range of 20-30%.
Forms of Power Resources
Dispatchable energy sources are those sources that can be turned on and off in a relatively short amount of time. This could refer to time intervals of a few seconds up to a couple of hours. Within the category of dispatchable power there are a lot of different technologies. On the fast end we have forms like hydroelectricity which can be fired up in minutes. On the slower end we have things like most biomass or coal plants, which can take hours to change their energy output significantly. Natural gas turbines are a very common dispatchable source, and they can generally be ramped up in minutes. We learned that SaskPower uses some gas turbines that stay spinning at relatively high speed but not producing power2. In this way they burn very minimal fuel but are ready for almost instant deployment in energy production.
For a more in-depth look at dispatchable power, see our article: How can renewables deliver dispatchable power on demand?
In contrast, non-dispatchable refers to everything else. This includes all current nuclear power plants, most coal power plants, and run-of-river hydroelectric plants. It also includes intermittent energy sources such as wind, solar photovoltaics, and wave energy. These power sources cannot be relied upon to meet demand in a short amount of time, so they are non-dispatchable.
The intermittent sources such as wind, solar photovoltaics, run-of-river hydroelectric, and waves, are those sources for which we do not control their power output directly. These are sources that cannot be relied upon to meet power demands. They should instead be regarded as an energy resource. The more the wind blows through our wind turbines, the less natural gas we have to burn, or the less water we have to run through out of our hydroelectric reservoirs. These intermittent sources can be used to reduce the amount of fuel we use for meeting demand with dispatchable sources.
Baseload in common usage refers to power stations that are always on and are generally the biggest generation units. If this term is used it generally refers to coal and nuclear power. Sometimes reservoir-based hydroelectric power sources are also included if they have large enough reservoirs or have proven reliability. These are generally the power sources used to meet most of the demand in an electrical system. These sources are always on unless they are down for maintenance, repair or refueling in the case of nuclear.
Cogeneration (also known as combined heat and power) refers to the use of waste heat from a thermal power plant to do other useful things. Coolant being expelled from an electric generation system may be still quite hot. Different uses of the waste heat are possible depending on the temperature of the available coolant, and the location of nearby industry or residences. In the case of industry nearby, the heat may be used for processes that require high temperature. The heat may also be used for heating buildings for industry or residential use. Cogeneration is essentially taking advantage of a natural synergy between thermal electric power plants and other uses for heat.
- Centennial Wind Power Facility Rides the Wind to a Great First Year. SaskPower, June 14th, 2007. Retrieved Sep 9th, 2010. [↩]
- Gary Wilkinson, SaskPower: Powering a Sustainable Energy Future, Saskatchewan’s Energy Future Public Consultation, Saskatchewan Legislature, available in the Standing Committee on Crown and Central Agencies Archives [↩]