Electricity Demand

According to the CIA World Factbook, the total worldwide electrical energy consumption in 2004 from all sources, including fossil fuels and renewables, was 15,406 TeraWatt hours (TWh). Such large numbers are difficult to envisage so the energy consumption is often expressed in terms of the amount of oil which contains an equivalent amount of energy, in this case 1,328 Million Tons of Oil Equivalent (Mtoe). This is just the demand for electrical energy. The total energy consumption for all purposes including oil and gas for heating and transport and other uses amounted to over 10,291 Mtoe in 2004 and 10,537 in 2005 (BP Statistical Revue of World Energy)

Total Annual Electrical Energy Demand by Country

The chart below shows the total electrical energy consumed by the world's top ten consumers, with scales in both TWh and Mtoe for convenience

Source CIA World Factbook 2006

As a nation the USA is the world's biggest consumer of electrical energy but on a per capita basis, the top ten national consumers are quite different.

Per Capita Annual Energy Demand by Country (For all applications)

The total national electrical energy consumption per capita for all applications (Not just domestic consumption) is shown below.

The UK comes number 41 on the list with 5,679 kWh

Source CIA World Factbook 2006

Only about 30% to 35% of electrical energy is used for domestic consumption, most of the rest is roughly evenly split between industry and commerce with between 1% and 3% being used in agriculture and for transport.

The chart opposite shows the usage of electrical energy by application in the USA. Similar demand patterns also apply in Europe and other countries.

Daily Domestic Electricity Demand by Country

The chart below shows the average daily per capita domestic electrical consumption for the top twelve consuming nations in Europe. The average household demand will be between 2 and 3 times these amounts.

Source "Energy Powering Your World" EFDA - European Fusion Development Agreement (2002)

The annual electrical energy consumption in most European countries averages about 2000 KWh per person (or 5.5 kWh per day) but ranges from 7467 KWh per person in Norway, where electricity is inexpensive and people use it for heating their homes, down to only 352 KWh per person in Romania where household incomes are much lower and electricity is relatively expensive. With an average household occupation of 2.6 persons this means that the typical European household uses about 14.2 KWh of electrical energy per day.

Comparable domestic consumption figures for the USA are 4387 kWh per capita per year or 12 kWh per person per day or 30 kWh per household per day.

A 1,000 MegaWatt power plant working with a capacity factor of 80% will thus provide enough energy for 640,000 households in the USA or 1.3 million households in Europe.

Domestic Electrical Energy Usage

The table below shows the electricity consumption of typical household electrical appliances

Load Patterns

The graph below shows a typical demand profile of an individual household monitored every two 2 minutes.

Individual Household Electricity Demand

Source: The Environmental Change Institute, University of Oxford (Modified)

It would be very difficult to match generating capacity to such a peaky demand profile. Fortunately the aggregate demand for all industrial and domestic consumers in a particular community tends to smooth out the overall demand profile and although the aggregate demand varies during the day and also over the course of the year, it does so in reasonably predictable patterns.

The graph below shows the total California load profile for a hot day in 1999

Aggregate (Community) Electricity Consumption

Source: Lawrence Berkeley National Laboratory

While the aggregate demand smoothes out the individual peaks, the daytime load is still double the night time load.

Small Stand Alone Generating Systems

Stand alone generating systems do not have the opportunity to even out demand by combining the requirements of several consumers. The solution for supplying intermittent peak loads is to use batteries to provide a buffer energy store from which energy can be withdrawn on demand thus decoupling the generator from the load.

Stand alone systems are often solar or wind powered and so may also have the problem of intermittent energy supply as well as intermittent demand. Using a battery buffer store enables the supply to be matched to the demand.

Load Matching

Electricity generating utilities generally have strict supply obligations requiring them to ensure quality and continuity of service at a reasonable price under all but the most extreme of circumstances. In terms of generating capacity, this means that the supply should follow the demand. Unfortunately the timing of the peak demand rarely coincides with the availability of the most convenient peak supply.

• Demand Characteristics

As noted above, overall demand follows regular patterns with a fairly large variation in demand over the course of the day. Superimposed on this daily pattern are smaller, longer term, seasonal variations with a greater demand for heating and lighting in winter months or in some regions for air conditioning in summer months. Demand from some major industrial users may also follow cyclic variations which could also affect the aggregate demand. Operating experience and statistics backed up by local knowledge about economic growth trends allow reasonably accurate predictions of demand to be made. On top of this, the utility is expected to cater for unexpected emergency situations, such as accidents, natural disasters or breakdown of its own equipment.

• Supply Characteristics

Generating capacity on the other hand tends to be fixed, particularly in the short term, since the planning, installation and commissioning of new capacity could take several years. This means that the installed capacity must be sufficient to supply the anticipated peaks in demand with the unavoidable consequences that it will be under utilised during the daily and seasonal periods of low demand and this inefficient use of capital will be reflected in the costs of the electricity.

Fortunately these costs can be mitigated by the diversity of generating options available which provide some flexibility to the planner.

 

• Planning Options
  • The Base Load

  The base load is usually carried by the newest, most efficient generating plants operating at their optimum capacity.

  • Power Control Systems

  Although the energy supply is controllable it can only be varied up to the rated power of the local system. If the demands are greater than the local system can deliver then other measures have to be taken.

  •  The Grid

  The electricity grid was set up to allow regions with surplus capacity to export their power to regions with a deficit. This could be a short term measure with one region supplying its neighbour in case of a temporary shortage or an emergency, or it could be a longer term strategy for regions with few or inefficient resources to purchase their energy form a more efficient, lower cost source. Long distance power transmission will of course be subject to resistive losses in the transmission network.

  • Intermittent Energy Sources

  In recent years an increasing number of renewable energy resources have come on stream. Although the availability of hydroelectric power is reasonably predictable and controllable, solar and wind power are only available when the weather permits. It is thus almost impossible to match this supply to the demand. The only way to incorporate these resources into the system is to provide alternative capacity known as spinning reserve for plugging the supply gap when the sun doesn't shine or the wind doesn't blow or to provide high capacity energy storage (batteries or alternatives) to buffer the supply, absorbing the supply peaks and releasing the energy in a controlled way over a longer period.

  • Spinning Reserve

  Spinning reserve plants provide generating capacity which can be brought on stream at short notice. They are needed to satisfy regular or daily peak demands which can not be met by the base load generators. Since they may be expected to have a very low load factor, older, less efficient plants are usually designated for this task.

  Spinning reserves are also essential for complementing the intermittent renewable energy sources as noted above.

  • Load Leveling

  Load leveling encompasses several schemes used for matching the supply to the load or vice versa. It involves moving demand from peak periods to times when demand is low, thus flattening the demand curve. Peak Shaving, the Smart Grid and Energy Storage are examples. See below.

  • Peak Shaving

  Power plants designed to provide emergency or opportunist power are called peak shaving plants. They are mostly gas turbine plants which can be brought on stream very quickly in case of sudden unplanned demand. Large industrial energy users who buy their electricity at "spot" rates, which tend to be higher when demand is high, may choose to use their own highly efficient modern generating plant at times of high demand when spot rates exceed their own generating costs, switching back to the grid when demand and rates fall. Using distributed generation in this way can take some pressure off utility's the peak energy demand, hence the name "peak shaving". It essentially a way that utilities can use to reduce their supply obligations by encouraging users to invest in incremental grid capacity which will be used at a very low load factor.

  • The Smart Grid

  Smart Grid it is actively promoted by the US Department of Energy (DOE) and aims to bring the same level of intelligence used to manage the telephone network to the nation’s power distribution network.

  Instead of matching the supply to the demand, the aims of the Smart Grid are, by contrast, to match the demand to the available supply by using technology and coercion through variable pricing in order to obtain better utilisation of capital assets. Consumers are encouraged to consume at specific times and to curtail their demand at others. It needs metering technologies to track consumption and software systems to inform the user of current pricing rates to allow them to modify their demand pattern.

  The main objective is to flatten the demand profile but it also allows pricing to reflect the availability or otherwise of power from intermittent wind and solar sources and to induce consumers to use it when it is available.

  For success the Smart Grid requires the implementation of a new level of intelligence in the distribution network with two way communications reaching down to the consumers’ premises.

  • Vehicle to Grid Systems (V2G)

    The V2G concept currently under serious consideration as a Smart Grid application, particularly in the USA, is to use suitably equipped private, passenger electric vehicles to provide a load leveling function for the electric utilities. The idea is to charge the vehicle's batteries at night when demand is low utilising the utility's excess capacity, so called "valley filling" and allowing the utility to draw energy from the battery, sending it back to the grid during the day when demand is high thus reducing the peak demand on the utility's capacity, by so called "peak shaving". It may sound very attractive in theory, particularly for the utilities, but a lot of things need to happen for it to be put into practice. It depends on the following assumptions:

    • The population (market share) of electric vehicles will be very high.
    • The vehicles will be EVs or PHEVs which normally have high capacity (20kWh to 50kWh) batteries, not HEVs which have much smaller (less than 2kWh) batteries.
    • An infrastructure of charge-discharge stations will need to be constructed at work places, car parks and private residences.
    • Charge-discharge systems (including domestic systems) must incorporate an inverter if AC is to be returned to the mains.
    • An intelligent computer network must be set up to monitor the charge in the batteries, to manage the energy flows and to ensure that the vehicles always have enough energy to get home.
    • An automated billing system needs to be set up.
    • Commuters will arrive at work with sufficient charge left in their batteries to make the idea feasible, otherwise they must specify oversize batteries which are already very expensive.
    • Users will need to specify suitably equipped vehicles and to install intelligent charging stations at home.
    • Users must accept that every charge-discharge cycle used by the utility will mean one less available to themselves for travelling and the available cycles will be shared with the utility on a pro rata basis to their use. This essentially reduces the user's effective battery lifetime and increases the user's capital cost per cycle used.
    • Sufficient financial benefits will be available to provide significant incentives to the users to get them to sign up.
    • Sufficient EV and PHEV owners will actually sign up to use the system.

    So far it should not come as a surprise that there's no queue of takers.

    
    See also Electric Vehicle Charging Infrastructure
    

     

  • Energy Storage

  While not adding to generating capacity, batteries allow surplus energy to be stored until there is an opportunity to use it. Several multi-MWh batteries using conventional battery chemistries have been installed in various countries, mainly to provide security of supply. Mobile batteries with capacities of 1MWh or more which can be accommodated in transportation containers have been used for emergency power applications.

  The recent growth of wind and solar power applications has created a demand for load leveling batteries to enable these intermittent power sources to deliver a continuous power to the grid.

  Round trip storage efficiencies for batteries are typically 80-90%. For high capacity batteries this amounts to a significant penalty.

  More methods of high capacity energy storage are given in the section on Alternative Energy Storage Methods.

  • Emergency Power Generation

  Low power emergency power is mostly provided by Piston Engined Generating Sets, while Gas Turbine Generators may be used for very high capacity emergency power.

  • Load Shedding

  As a last resort, when demand exceeds supply, it may be necessary to reduce the demand by disconnecting some users.

See also Plant Utilisation Efficiency

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