|
 |
| |||||
|
|||||
|
|||||
|
|||||
|
|
|
Wind energy, total domestic energy needs, 'green' electricity and electricity supply. What is the difference between kilowatts and kilowatt-hours?
Watts - Power generation/Power consumption capacity If you go back to the text-books you will find that a watt is the unit rate at which work is done in an electrical circuit and the definition commonly applied to it is that one watt is equivalent to; The Watt is a pretty small unit of power as a result of which and in order to talk about meaningful amounts, there is very soon a need to refer to thousands, millions, billions or even trillions of watts. Rather than use lots of zeros in such situations, industry practice is simply to use prefixes and these are (together with their corresponding acronyms); kilowatt (kW) = 1,000 watts Watt-hours - Power generated/Power consumed. Now let us assume that the pump has just been used to remove water from a large hole in the ground over a period of one hour. However knowing the pump's capacity (1 litre/second) does not tell us how much water has been moved by it in the hour. The situation with a piece of electrical machinery is similar - just because we know its capacity for work or power rating (watts) we do not know how much work it has done in any period of time. Enter the watt-hour. Going back to the electrical example: we might find that the average house in a typical town in New Zealand has a fridge, a freezer, a water heater and an oven. Let us also assume that the total power needs of these items, if they were all turned on at the same time, is 10kW or 10,000 watts. Unfortunately this does not tell us how much electricity is actually consumed over the course of a day, a month, a year or longer by this hypothetical house. To solve this problem we turn to the watt-hour. One watt-hour is equivalent to one watt of power consumed (or generated) for a period of one hour of time. The same prefixes are used with watt-hours as are used with watts: i.e. one kilowatt hour (kWh) = 1,000 watt-hours If you put all of this into a slightly broader context: New Zealand's total electricity consumption in 2002 was 33.14 TWh. This is equivalent to the amount of power which would have been generated by a 3,700MW generator running for 100% of the time throughout the year. This is not as fanciful as it might sound - there is in fact a single power station in the UK (called Drax) which has an installed capacity of about 3,900MW. What are PetaJoules and how are they connected to watts? one kWh = 3.6MJ (mega joules) Since 1GWh = .0036PJ we know that 30PJ = 30/0.0036 = 8,333GWh. What does this mean in terms of installed capacity of, for example, wind turbine generators? From the example below we see that a single one MW turbine can, using conservative estimates, generate 3.5GWh annually. Consequently, if the entire NEECS target was to be met using power generated from wind turbines, one can see that New Zealand would need to install something in the region of 8,333/3.5 = 2,380 1MW turbines throughout the country. How much electricity can a single wind powered generator produce? The electricity that turbines produce when measured against generating capacity is referred to as the capacity factor. In some parts of Europe this may be in the low twenties. New Zealand boasts the highest capacity factors in the world. Wind turbines here have an average capacity factor of 41%, but in some locations the capacity factor of individual turbines can be as high as 55% or more. Using this information it is relatively easy to make a rough approximation of the amount of electricity that a single 'average' turbine can produce in a year.For example: a turbine with a rated capacity of 1MW and operating with a conservative capacity utilisation factor of 40%, will generate 1*0.4*8,760 MWh = 3,504 MWh annually (note that 8,760 is the number of hours in a year). What happens when the wind is not blowing? Electricity systems have evolved to cope with significant fluctuations in demand through both the day and the year. Generating capacity on any system therefore has to be capable of ensuring that it can respond to these fluctuations. Relative to these demand side fluctuations those which would occur due to changes in output of wind turbines are relatively minor. It is also very important to note wind power and hydro are perfectly suited to each other. For New Zealand this is very fortunate as more than 60% of our electricity currently comes from hydro dams. The reason that wind and hydro are well suited to each other is that when the wind is blowing the volume of water flowing through hydro dams around the country can be reduced. In effect what this means is that the electricity generated by wind turbines can be stored as potential energy in water in hydro dams. When the wind is not blowing this 'stored' wind energy can then be released and used to generate electricity. In an article in the NZ Herald in February (see our News section for details) the past president of the Institution of Professional Engineers of New Zealand and Director at the Center for Advanced Engineering at the University of Canterbury (John Blakeley) talks about the potential solutions to New Zealand's power needs. He calls for "a very substantial and urgent programme of constructing wind farms" and states that "we can use the storage capacity of our hydro lakes to balance fluctuating wind power generation". Various studies have been carried out around the world which have sought to determine the limit beyond which installing intermittent capacity (i.e. wind generators) starts to incur significant costs in order to maintain the stability of the electrical grid. The level varies but is typically 15-30% of the total installed capacity. Since New Zealand has a total installed generating capacity of 8,412MW this 15-30% band would be equivalent to 1,200-2,400MW of wind turbine generators (WTGs). In the Herald article referenced above, John Blakeley mentions a limit of 1,000-1,500MW of WTGs. Whatever the exact number it should be reasonably clear that, relative to the amount of wind capacity currently installed (72.6MW), we have a long way to go. It is nonetheless clear that we do need to determine the maximum theoretical limit for the penetration of intermittent wind energy on our grid. In order to do this it is important that an independent study into this question is commissioned as soon as possible. It is equally important that any such study involves all stakeholders and that it has clearly defined and commonly agreed, terms of reference. Without this the results of the study will be subject to debate and its value will therefore be limited. Where can I find information on domestic energy consumption? See the section below (Where can I find reliable statistical data on energy? - Domestic - total energy) How can I ensure that the electricity I buy at home is generated from renewable sources of energy? Some countries in the world have independent electricity retailers who will sell 'green' (100% renewably generated), brown (non-renewably generated) electricity or indeed a mix of these types. In New Zealand we do not have such independent retailers and instead electricity must be purchased direct from the electricity generators. Consequently if you are seeking to ensure that your electricity is generated renewably, you will need to look at the mix of generation assets of the generator from whom you buy your electricity. Greenpeace has recently compiled a guide which seeks to rank each electricity generating company according to its environmental impact both now and in the future. Details are available at www.CleanEnergyGuide.org or by clicking on the picture below.
Where can I find reliable statistical data on energy? Alternatively you can download MED's July 2003 datafile (  229kB)
Global - Wind Energy: You can find the most recent global wind energy installed capacities on the NZWEA web site; alternatively you can access the Global Wind Energy Council press release of 2004 statistics ( Global - total energy: For information on global hydrocarbon production and consumption the BP Statistical Review of World Energy has been an authoritative source of information for more than half a century. This is an excellent web site with downloadable spreadsheets containing a wealth of data on reserves, production and consumption for every significant country in the world. It also has conversion factors for the multitude of different units that are in use in the energy sector as well as an extensive glossary for interpreting a wide array of energy related acronyms and abbreviations. The review is all the more interesting as BP have recently added a renewable energy section which includes a breakdown of renewable sources of power. Follow the links to the 'Statistical Review of World Energy' from the BP home page (www.bp.com). You may also wish to try the International Energy Agency in Paris who have an area of their site dedicated solely to world energy statistics (Details) How do electricity prices in New Zealand compare with those elswhere in the world? Because of the peculiarities of the rapidly depleting Maui gas field, and the way in which its costs were passed on to consumers, New Zealand has, until recently, enjoyed some of the cheapest retail electricity prices in the world. As a result conservation of energy, fuel efficiency and domestic insulation have all been low priorities. Recent increases in power prices, associated with the rapid decline the output from the Maui gas field, are starting to change all that. Graphic of NZ power prices, compared internationally, prepared at the beginning of 2004 (
|
