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Contribution of combined heat and power (CHP) plants to UK energy usage consumption of primary energy.
CHP is a front runner for the modern way of utilising fossil fuels efficiently. The aim of this report is to highlight the UK’s energy usage in terms of primary and end user energy; by doing so we can analyse the amount of energy which is used by each sector and thus has an idea of how CHP could impact our economy. CHP has tremendous benefits on over conventional energy production methods, it is therefore important that the system is fully understood. The system, has a variety of important components which allow it to function efficiently; these components are examined in the report. CHP follows the first and second laws of thermodynamics which is an important concept to understand, as the reasoning for how the heat in the system get used and recovered.
Table of Figures
Figure 2: A Comparison between CHP & Traditional Heat and Power Production (Creative, 2016)…9
This report will explore the benefits of a CHP system by analysing the current method of distributing energy around the UK. The report will also investigate the current UK energy usage, to conclude the benefits which CHP could have. The components, advantages and disadvantages of the system is also analysed, in order to better grasp the impact which the system could have. The thermodynamic properties are also analysed to gain an understanding of the principles which the system follows.
In order to fully grasp the potential which CHP could have on the UK’s economy, this document includes an in depth report on the current UK’s energy usage and the predicted usage for the forthcoming years. In order to perceive a magnitude of the current UK’s energy usage the report will analyse the UK’s energy usage over the past 50 years. The report will also analyse the usage of each sector and compare this to the growth in population; to determine whether we are more energy dependant as individuals. The methods used in this report will not only enable for a better and more detailed analysis; but it will also allow to emphasize where and how CHP may have the biggest impact. To fully understand the workings of a CHP plant output, we must look in detail at all the constituent elements of the plant, and understand the motive of each of them. Furthermore, we must understand how the CHP plant itself links to thermodynamics principles as this will allow us to grasp how efficient the system is and could potentially be. To understand the thermodynamic properties of the system, an analysis was made on the components to comprehend what they do. After carrying out all of these analysis, the pros and cons of the system can be justified and a conclusion can be deduced.
Primary energy usage
As seen in Figure 1, the majority of the UK’s energy source has been generated by Gas, Oil and Coal with a small percentage generated by alternate sources. The graph shows that since 2005 there has been an increase in the amount of energy which is sourced by bioenergy and wind energy whilst the reliance on gas, oil and coal has been slowly diminishing, along with the need to rely on imported energy.
“Total UK GHG emissions have dropped by 35% since 1990 and Total UK carbon dioxide emissions have decreased by around 29% since 1990.” (Visual.ons.gov.uk, 2016)
As a result of the decreased dependency on fossil fuels, as stated by (Visual.ons.gov.uk, 2016) it has had a direct effect on the amount of emissions produced in the UK. It is fair to say that this has a direct correlation to the amount of energy which is sourced by renewable energy. Figure 5 in Appendix 1 shows the amount of energy which has been produced by renewable sources over the past 16 years, which shows a direct relationship with the decline of fossil fuel dependency.
The trend in the reduction of the UK’s energy consumption can be traced to the government’s aim to increase the amount of energy sourced from renewable avenues. “15% of energy consumed in the UK should come from renewables by 2020” (Full Fact, 2016). The target has been set as a result of the increasing temperature change which is causing global warming. Whilst the increase in temperature is a worry, the burning of fossil fuels is also having a damaging effect on our environment (causing the greenhouse effect). According to (Carbon Brief, 2016) Carbon Dioxide has risen by 38% over the last 250 years and would has had a detrimental effect to the planet in the forthcoming years if something wasn’t done about it. The steps made by the UK and the rest of Europe has a positive effect is reversing some of the damage.
FIGURE 1. UK PRIMARY ENERGY USAGE (GOV.UK, 2016)
UK energy usage compared to rest of Europe
In order to truly understand the overall UK’s energy consumption, it is important that the overall energy usage is compared on a global scale and not solely on a historic level.
According to (Indexmundi, 2016) the UK’s population is 2014 was 64 million, whilst France was 66 million, therefore France will be used in this example to draw a comparison.
According to (Yearbook, 2016) in 2015 the UK used 179 Mtoe of energy whilst France used 249 Mtoe. This is a staggering difference in total energy usage which could be accounted to a number of reasons. “The EU has set a 15% overall target for UK energy consumption from renewable sources in 2020.” (Full Fact, 2016). The target set by the EU is a major driving force for the decline in the UK’s energy consumption this is reflected in the fact that in 2013 the UK and France used 191 Mtoe and 253Mtoe respectively (Yearbook, 2016). Which therefore has resulted in a 4% combined decrease from 2013 to 2015. On the grand scheme of things, the UK is not only decreasing its amount of energy usage (by depending on alternate renewable sources); but it is also consuming less energy relative to its population than other countries in Europe. This can further be emphasised when comparing the percentage, the UK’s energy usage in Europe 10% to France which is 14%. The trend can be observed from Figure 6 in Appendix 2.
End user energy usage
In order to gain a better understanding of the UK’s energy usage; it is important to analyse the energy consumed in each of the major sectors, these include transport, domestic, industrial and services. By analysing the energy trend used in each of these sectors an assumption can be made about where most of the energy is used and also what can be done to improve the way in which energy in consumed
The transport sector is an important area of final usage to analyse as it is one of the major areas which thousands of people are dependent on it in their daily lives. The transport sector accounted for roughly 40% of the energy consumption in 2015 (Visual.ons.gov.uk, 2016). If we take into account, the total energy use of the 5 main sectors (179Mtoe) the transport sector uses a staggering amount of our overall energy each year and if some could be salvaged the benefits can be both financial and environmentally beneficial.
“Final energy consumption in the transport sector increased by 684 ktoe (1.3 per cent) between 2014 and 2015 to 54,810 ktoe; the second year of positive growth following the economic slowdown” (Gov.uk, 2016).
More people each year are resorting to public transportation as their primary source of commuting. This growing trend is seeing a major increase in the amount of energy consumption in this sector. The amount of energy used in the transport sector has almost doubled in the last 40 years (appendix 3) and shows no signs of stopping a shown in Figure 9 in Appendix 2. As a result of the amount of energy which is consumed by this sector the mayor of London has reviled plans to have “300 single-deck buses that travel through central London to be zero emission by 2020, and all 3,100 double deck buses to be hybrid by 2019.” (“Mayor Unveils First Fully Electric Bus Routes For Central London | London City Hall”). If these plans were replicated by the rest of the cities in the UK; with CHP being the primary form or our energy source, the total transportation energy consumption could by slashed by 20%.
The domestic sector is the second largest sector in the UK in terms of final energy consumption. According to (Gov.uk, 2016) the final consumption has increased by 3.6 percent from 2014-15, or 1% when temperature is taken into account. Comparing the growth in domestic energy usage of around 1% to the growth in population of around 0.6% (Figure 10 in Appendix 3). It is safe to make the assumption that as a population the UK is using more energy per person. The growth in usage of energy per person could be a result of the reliance on technology on a day to day basis per capita. This is a growing trend, which suggests that we have to begin to source out energy from alternate sources to reach the target to reduce energy consumption.
The cost of gas and electric has more than doubled since 2002. The rise in energy is a due to a number of factors, one of which being the rise in the wholesale energy cost. Taking these factors into account, having CHP as our main source of energy plants would in effect save the end user money on their bill. According to (Burns) CHP could save the user up to 40% on their energy bill.
The industrial sector is an important of the UK’s economy as it creates jobs, skills and wealth. In order to keep the Industrial sector thriving it requires 17% of total final energy consumption. Although this is a large percentage of the UK’s energy usage, it has been falling in this sector consistently since the 1970s; this trend can be analysed in more detail in Figure 8 in Appendix 2. This decline is a result of a lot of the main sub-industries (such as; the manufacturing) closing down over the years and thus less energy dependant industries coming to the forefront. Another reasons for this decrease, is due to the decrease in energy intensity in this sector. Energy intensity has decreased by 38 per cent between 1990 and 2015 in the industrial sector (Gov.uk, 2016).
The service sector is a thriving section of the UK’s economy, which accounts for 14% of the UK’s energy usage in 2015 (Gov.uk, 2016). The major subsectors which requires the most energy are; retail, hotel and catering, education, and warehouse with the other sectors not falling far behind. It is not surprising that these sectors are carrying the baton in terms of energy usage as they are at the forefront of what drives the UK’s economical infrastructure. As this sector is a stable part of the UK’s economy, that stability can also be seen if analysing the final energy consumption over a long period of time. Figure 7 in Appendix 2 shows the fluctuation in final energy consumption but it also outlines the stability in the overall final energy consumption; which has fluctuated around the 20,000ktoe mark since the 1970’s.
What is Combined Heat & Power (CHP)?
Combined Heat & Power (CHP) also known as cogeneration, combines the production of heat and electricity into a highly efficient process compared to the conventional methods of producing these commodities. The production is from a single source which occurs at the same time. This then increases the overall useful energy production from around 56% to 80%. (Creative, 2016).
The conventional way that a power plant produces electricity is a fairly inefficient process with fossil fuels such as oil, coal and natural gas being burned in a furnace to release the energy. This energy that is released is in the form of heat, this is then used to to boil water which then creates steam. This steam drives a turbine which powers a generator, thus producing electricity. However, at every stage of this type of energy production energy is wasted, such as the water has to be cooled down using large cooling towers, allowing a large amount of heat energy to be wasted in to the air. The main idea behind CHP is to capture the by-product of this type of power production and then supply it to local offices and homes, as hot water. (Woodford, 2016)
How does CHP work?
FIGURE 2: A COMPARISON BETWEEN CHP & TRADITIONAL HEAT AND POWER PRODUCTION METHODS (CREATIVE, 2016)
CHP systems can replace the more common 2 separate systems that are used to produce heat and power, that would use two separate fuel inputs which would give twice the amount of wasted energy compared to the one that at CHP Plants would waste. This therefore makes CHP more efficient compared to the more common systems. The image below demonstrates this fact as for the same fuel input the CHP Plant is clearly more efficient compare to the traditional method.
The main component of a CHP system is called a prime mover or a heat engine, these parts provide the power to generate the electricity that produces the heat. These components are usually a combustion engine, a gas turbine or a steam turbine. This range of prime movers means that the fuel used to produce the electricity and heat can vary depending on demands i.e. hot water or steam.
The heat energy created by the CHP system is captured using a heat recovery system, this then removes the need for a secondary turbine to generate more electricity thus making the CHP system more efficient or another boiler used for heating space and water in the local area. CHP can be used in all buildings regardless to their size, this means is can be used in Homes, Hospitals and Schools through to high energy consuming power plants. (Carbon Trust, 2011)
Advantages and Disadvantages of CHP
The main purpose of CHP is to improve the efficiency of the initial fuel supply; this is then the main advantage of CHP. This then means that there will be a reduction in expenditure on the fuel supply, while also reducing the carbon emissions in the UK as the useful energy can be achieved with a lower amount of fuel being used.
With energy prices in the UK rising the implementation of CHP could help reduce these prices as the companies would be spending less on fuel to produce the same amount useful energy. The amount that the companies save could then passed on to the consumer, however this would be for a centralised system. However, if Micro-CHP systems were installed in domestic dwellings this could reduce annual energy bill by a minimum of 10% but could be higher depending upon the technologies that are installed in to the dwelling. (Creative, 2016)
CHP can also be used in remote areas where the national grid isn’t connected, however is this suitable for Micro-CHP. For this type of set up, the fuel used is usually LPG Gas, which is readily available thus meaning easier to replace when empty. This kind of system can be up to 90% efficient with maintenance costs being around the same as a conventional boiler. (Energy Saving Trust, 2016)
Grants are available from the Department for Business, Energy & Industrial Strategy, which provides an incentive for businesses as they given tax breaks and be business rate exempt depending upon on the quality of their CHP system. There are also incentives for domestic applications of CHP, as the government have a scheme where the owner of the CHP system can sell back their excess energy to an energy company. (Department for Business, Energy & Industrial Strategy, 2008)
A disadvantage of CHP systems within the UK, is that heat and power have to be produced at the same time to ensure that the system is working efficiently. So during the summer the heat energy produced would be wasted whilst the power consumption would remain constant. However, this can be solved by using tri-generation otherwise known as Combined Cooling Heat & Power, this process uses absorption cooling from the heat source to deliver cooling rather than thermal energy. (ESRU, N.D)
As the technology used in a CHP System is quite complex, maintenance must be carried out on a regular basis by an expert. This can cause problems due to the fact the technology is quite new, meaning that the number of experts is small so this would make the maintenance and repairs of the CHP system quite expensive. Also parts for these systems would be in limited supply making them expensive thus increasing the payback time of the system. (Enviko, N.D)
Major flaw in CHP systems is that in a centralised infrastructure, it would still be reliant upon fossil fuels. This could then mean that more financial resources are spent developing energy production for non-renewable sources, this could to lead to a reduction in research and development for other energy sources, thus making the UK’s population more reliant upon fossil fuels. (ESRU, N.D)
The Thermodynamic Principles which work in CHP’s
First Law of Thermodynamics
The First Law of Thermodynamics essentially the conservation of energy applied to real world thermodynamic systems, it states that the change in internal energy is directly equal to the heat added to the system less that of the work done by the system. This can be represented by the equation in Figure 3;
FIGURE 3. FIRST LAW OF THERMODYNAMICS EQUATION (HYPERPHYSICS N.D)
It is commonly noted in Chemistry as the addition of Q and W, as opposed to the Physics version of Q minus W, the only difference then is that by definition, W is no longer defined as work done by the system, but rather, work done on the system (HyperPhysics N.D)
Second Law of Thermodynamics
The Second Law of Thermodynamics is a generalisation upon the fact that there can be no perfect engine despite how well build and efficient it is. The efficiency in which we model our engines on now, but can never truly reach it is the Carnot Efficiency.
FIGURE 4. VISUAL REPRESENTATION OF THE SECOND LAW OF THERMODYNAMICS (HYPERPHYSICS N.D)
In the visual representation above, if TL (Sink Temperature) was lower than 300K, then this would increase the Thermal Efficiency as this is defined by, 1 – Qout/Qin = 1 – QC/QH (using the terminology from Figure 11 in Appendix 4). From this we can deduce that the Carnot Efficiency would be if the sink temperature was 0K meaning 100% of the energy would be converted to useful work output, the Thermal Efficiency would be 100%, this can be further backed up by using the equation stated above. Thermal Efficiency = 1 – 0/600 = 1 = 100% would be the proof that if the sink temperature was 0K, Carnot Efficiency would have been achieved. The Second Law of Thermodynamics states that this is impossible. As you can see in Figure 11 in Appendix 4 is it impossible for a system to reach Carnot Efficiency and it is therefore one of the limitations of systems as a whole in the real world. Taking this into account, by using the sink as the means for a new source in a system, this reduces the overall loss and increases the efficiency, almost like a cascaded system similar to that in the refrigeration cycle.
Despite the fact that CHP systems aim to increase efficiency by converting energy, even this is restricted by the Second Law of Thermodynamics meaning it cannot be 100% efficient. It is imperative to understand that CHP is restricted by both the First and Second laws.
Components of CHP Power Plant
Figure 12 found in Appendix 4 shows visually the basis of what a CHP power plant aims to do. By generating both Electricity and Heat at one plant, this will increase the efficiency from 58% to 85%. Typically, a CHP system will have a few fundamental components that are present in all variations, these components include;
- Gas-fuelled reciprocating engine
- Heat Recovery system
- Digital Control System
- Acoustic Enclosure
- Prime Mover
Figure 13 found in Appendix 5 shows one possible way in which a CHP plant could be set up, the Figure includes a colour coordinated key. There are many ways in which a CHP power plant could be set up and Figure 13 only shows one example.
The CHP system itself needs some sort of engine to run, or something of a similar nature. The system inside the CHP plant is called the prime mover. The market as of 2012 according to the online source, CIBSE CHP Group say that available on the market. The prime movers that are available are; reciprocating engines (5kW-20MW), fuel cells (1kW-10MW), micro turbines (25kW-500kW), steam turbines (50kW+) and combustion turbines (500kW- 100MW). Depending on the prime mover that is installed into the CHP power plant, this can be powered by a multitude of things. These power sources include; diesel and petrol, biofuel, hydrogen fuel cells, coal and oil and the most common – natural gas (CIBSE CHP Group, 2012). Due to the variety of prime movers available, much care is taken when choosing which prime mover to use as they all offer individual advantages and disadvantages. However, the feasibility, size, thermal and electrical load profile, Carbon Dioxide emissions, cost, sound and start up time are usually the main factors which are taken into consideration when choosing the prime mover (CIBSE CHP Group, 2012).
Heat Recovery Systems
Heat recovery systems are a crucial part of any CHP plant, this is due to the fact that it captures the heat which has been produced from the prime mover unit and will in effect, recover the heat and then use this to heat or even boil water (Carbon Trust, 2010). Typically, a heat recovery system will consist of a plate heat exchanger. This is opposed to a gas turbine which uses a system known as HSRG (heat system recovery generator), however, a disadvantage of this is that sometimes the HSRG needs additional fuel to run which adds costs to the CHP plant. This is not known as HSRG but ‘fires HSRG’ (Carbon Trust, 2010).
When in a steam turbine the heat which is produced from the prime mover is usually directly used. In a steam turbine, the pressure is very high so before using a heat recovery system, the pressure may need to be slightly reduced.
With regards to the fuel cells which can be implemented in CHP plants, Woking Council (Carbon Trust, 2010). Woking Council provide the UK’s largest CHP’s scheme. Generally, a fuel cell is a cell powered by an electrochemical source which generates some heat by oxidising the fuel for it. A catalyst is used sometimes, but fuel cells which operate at a greater temperature than 600 degrees Celsius do not need this catalyst (Carbon Trust, 2010). Fuel cell systems are more expensive than other engines but as a result, they offer around a 55% increase in the electrical efficiency (Carbon Trust, 2010).
Steam turbines are based on a high-pressure steady stream of steam which is then in turn used to drive a turbine, this high pressure steam is generated from a boiler. One way to increase the efficiency is to condensed and then pumped straight back in to the boiler as high temperature water or even sometimes only a few degrees below the boiling point. The closer to the boiling point the water is re-pumped in as, the higher the efficiency. The cycle which the steam turbines used is known in thermodynamics as the Rankine Cycle (Carbon Trust, 2010). From a CHP viewpoint, they have a typical electrical efficiency of around 10- 20% and the overall efficiency from around 77-82% (Carbon Trust, 2010).
Gas turbines are based on a steady stream of fuel (usually gas hence the name gas turbines) to power a turbine as opposed to the steam turbine which uses steam hence steam turbine. The power used for the gas turbines are larger than 1MWe, however, similarly to the fact that micro-CHP is available, there are also ‘mini’ versions of gas turbines which are powered on between 80kWe-100kWe (Carbon Trust, 2010). On the opposite side of the spectrum, there are also larger build of the gas turbines which run at about 100MWe. The efficiencies are as follows; around 21% for the mini gas turbines, 25% for the normal sized gas turbines and around 36% for the larger build of the gas turbines (Carbon Trust, 2010). Gas turbines have a higher efficiency than all steam turbines due to the fact of their higher temperature operation but require a ‘cleaner’ fuel which is natural gas. However, they are not perfect as they have a lower electrical efficiency that normal internal combustion engines but on the up side, they require less maintenance (Carbon Trust, 2010).
Combined Cycle Gas Turbines (CCGT)
Combined Cycle Gas Turbines otherwise known as CCGT are a form of gas turbines and steam engines. They use the high temperature exhaust deposits to generate a high pressure steam (similarly to the steam turbine) which generates more power. The combination of both the gas and steam turbines allows for a higher power efficiency of around 55% as maximum but an average of 52% (Carbon Trust, 2010).
The majority of the prime movers used are actually reciprocating engines otherwise known as internal combustion engines. They have an electrical efficiency of around 25-50% which makes them a rather economical option for a CHP plant. Several types of reciprocating engines are used and these use such cycles as the Diesel Cycle and the Otto Cycle (Distributed Generation, 1999). The sizes of reciprocating engines are extremely flexible and can be found in sizes for CHP plants for industry or even micro-CHP plants for the domestic user. As found by practice, reciprocating engines have proven to have a long lasting life and are reliable in particular, the diesel engines and block engines (Distributed Generation, 1999). With regards to maintaining a reciprocating engine, they require a periodic replacement of the oil for the engine itself, also a spark plug check and maybe replacing the coolant, this routine check and change takes place around 500-2000 hours (Distributed Generation, 1999).
Micro CHP is essentially the same concept as a full-sized CHP power plant. The main difference is that when using a full-sized power plant, the energy produced would usually have to go through a third party (National Grid), this is eliminated with the use of a micro- CHP plant. It is scaled down for domestic use. “The main output of a micro-CHP system is heat, with some electricity generation, at a typical ratio of about 6:1 for domestic appliances” – (Home, Energy and Electricity, 2016). This is a benefit for households as if this is also acts as a form of electricity generation, then any extra electricity generated from the house which is not used by the household itself can be sold back to the National Grid or stored in the house for future use.
Figure 14. In Appendix 5 shows in a basic manor how a micro CHP system could work, also the dimensions are given. “Units are around 1.5m long, 1m wide, and 1m deep, and weigh around 750kg” – (Explain That Stuff). The dimensions given are not that wide, allowing it to be stored in the house easily possibly near the boiler or in a separate concealed room, the weight of 750kg is not that bad relative to the work it can do. From this we can deduce that it is slightly bigger in dimensions than that of a standard boiler.
One disadvantage of using a micro CHP power plant for domestic use is, as indicated by the name, is a micro version. Meaning the constituent parts that make up the appliance are considerably lower in size and packed together more tightly to produce a considerably low output in comparison.
Saving energy by using CHP
CHP has the capability to reduce end user energy by 20-30% if the technology is used efficiently. This is achieved by reusing the heat which would usually be discarded and considered waste. A CHP system uses less fuel per unit of energy produced, which in turn creates less greenhouse gasses. The money saved by using a CHP system over the conventional can be reinvested into renewable energy and thus reducing energy consumption further. As mentioned briefly in the components of a CHP power plant section CHP is capable of being used in a magnitude of sectors, such as large scale (industrial & commercial), small scale and micro (homes). The adaptability of the technology gives it the capacity to positively impact multiple sectors and therefore making it more likely to reduce the UK’s energy usage as a whole, in order to match the EU’s environmental agenda (EU ETS).
After analysing the current UK’s energy usage, it clear to see that there is an increasing trend in the amount of energy consumed; in both primary and end user sectors. Taking the UK’s future energy targets into consideration, there has to be a viable, alternate source of energy which the UK’s can rely upon. After analysis on the thermodynamic properties of a CHP system, it becomes apparent that the benefits which CHP offers outweighs the negatives. If CHP systems were implemented correctly it could have a big impact on the UK’s economy, environment as well as financial benefits.
The UK currently relies upon fossil fuels as the primary source of energy, a source which is also has detrimental side effects. There are numerous arguments to support CHP becoming a primary energy source, and until renewable energy become a frontrunner; CHP is the best option for salvaging a flawed system.
As authors, we believe that CHP as a whole is a viable option for the UK to save energy and is a great improvement to the current energy production. As outlined earlier in the report, naturally, CHP has its own disadvantage; and before CHP would become the main source of energy in the UK, the disadvantages would have to be meticulously analysed and tried to be solved. All this having being said, we still firmly believe that CHP has the potential to become one of the main source of energy for the UK.
FIGURE 5: ELECTRICITY GENERATION BY MAIN RENEWABLE SOURCE SINCE 2000 (CARBON BRIEF, 2015)
FIGURE 6: UK AND FRANCE ENERGY CONSUMPTION (YEARBOOK, 2016)
FIGURE 7: ENERGY CONSUMPTION IN THE SERVICES SECTOR (GOV.UK, 2016)
FIGURE 8: TOTAL INDUSTRIAL CONSUMPTION BY FUEL MIX (GOV.UK, 2016)
FIGURE 9: UK TRANSPORT ENERGY USAGE (GOV.UK, 2016)
FIGURE 10: UK POPULATION AND YEARLY POPULATION GROWTH RATE (WORLDOMETERS, 2016)
FIGURE 11: VISUAL REPRESENTATION OF THE IMPOSSIBILITY OF CARNOT EFFICIENCY AS RESTRICTED BY THE SECOND LAW OF THERMODYNAMICS (HYPERPHYSICS, N.D)
FIGURE 12: VISUAL REPRESENTATION OF A COGENERATION SYSTEM (BIOENERGY, N.D)
FIGURE 13: EXAMPLE LAYOUT OF A CHP PLANT (VADOGROUP, N.D)
FIGURE 14: HOW DOES MICRO CHP WORK? (EXPLAIN THAT STUFF, N.D
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