Combined Heat and Power Systems

Combined Heat and Power (CHP), or Cogeneration, is the sequential or simultaneous generation of multiple forms of useful energy (usually mechanical and thermal) in a single, integrated system. In conventional electricity generation systems, about 35% of the energy potential contained in the fuel is converted on average into electricity, whilst the rest is lost as waste heat.

CHP systems uses both electricity and heat and therefore can achieve an efficiency of up to 90%, giving energy savings between 15-40% when compared with the separate production of electricity from conventional power stations and of heat from boilers.

CHP systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. The type of equipment that drives the overall system (i.e., the prime mover) typically identifies the CHP unit. 

Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical energy.

CHP Technology Options

Reciprocating or internal combustion engines (ICEs) are among the most widely used prime movers to power small electricity generators. Advantages include large variations in the size range available, fast start-up, good efficiencies under partial load efficiency, reliability, and long life.

Steam turbines are the most commonly employed prime movers for large power outputs. Steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy. System efficiencies can vary between 15 and 35% depending on the steam parameters.

Co-firing of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing dependence on fossil fuels. Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Most forms of biomass are suitable for co-firing. 

Steam engines are also proven technology but suited mainly for constant speed operation in industrial environments. Steam engines are available in different sizes ranging from a few kW to more than 1 MWe.

A gas turbine system requires landfill gas, biogas, or a biomass gasifier to produce the gas for the turbine. This biogas must be carefully filtered of particulate matter to avoid damaging the blades of the gas turbine.  

Stirling engines utilize any source of heat provided that it is of sufficiently high temperature. A wide variety of heat sources can be used but the Stirling engine is particularly well-suited to biomass fuels. Stirling engines are available in the 0.5 to 150 kWe range and a number of companies are working on its further development.

A micro-turbine recovers part of the exhaust heat for preheating the combustion air and hence increases overall efficiency to around 20-30%. Several competing manufacturers are developing units in the 25-250kWe range. Advantages of micro-turbines include compact and light weight design, a fairly wide size range due to modularity, and low noise levels. 

Saudi ARAMCO's CHP Initiatives

Recently ARAMCO announced the signing of agreements to build and operate cogeneration plants at three major oil and gas complexes in Saudi Arabia. These agreements demonstrate ARAMCO's commitment to pursue energy efficiency in its operation. Upon completion, the cogeneration plants will meet power and heating requirements at Abqaiq, Hawiya and Ras Tanura plants. These plants are expected to generate a total on 900MW of power and 1,500 tons of steam per hour when they come onstream in 2016.

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Waste-to-Energy Pathways

Waste-to-energy is the use of modern combustion and biological technologies to recover energy from urban wastes. The conversion of waste material to energy can proceed along three major pathways – thermochemical, biochemical and physicochemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. On the other hand, biochemical technologies are more suitable for wet wastes which are rich in organic matter.

Thermochemical Conversion

The three principal methods of thermochemical conversion are combustion (in excess air), gasification (in reduced air), and pyrolysis (in absence of air). The most common technique for producing both heat and electrical energy from wastes is direct combustion. Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.

Combustion technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine. Plasma gasification, which takes place at extremely high temperature, is also hogging limelight nowadays.

Biochemical Conversion

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biofertilizer and biogas. Anaerobic digestion is a reliable technology for the treatment of wet, organic waste.  Organic waste from various sources is biochemically degraded in highly controlled, oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat.

In addition, a variety of fuels can be produced from waste resources including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking.

Physico-chemical Conversion

The physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation. The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter are then mechanically separated before the waste is compacted and converted into pellets or RDF. Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.

Introduction to Solar Pond

A solar pond is a three-dimensional, open-air pit, filled with water endowed with special properties. It receives solar energy through insulation, then the trapped heat is extracted from it from the water lying at the bottom of the pond. When solar energy falls onto the pond, it heats the water, splitting it into three sections: the first section is the uppermost layer, or Surface Zone, containing fresh water with a low level of salinity. This owes to the fact that salts gather at the bottom.

The second layer is the middle layer, called the insulating layer or Insulation Zone, whose salinity is greater than that of the surface level. The most important layer, though, is the bottom or lowest layer, known as the Storage Zone. This is the layer which retains solar energy and at which the extraction of energy is possible. This saturated layer is between approximately one and two metres thick, whereas the pond is generally two or more metres deep.

When the water of any Solar Pond gathers heat, it expands, becomes less dense, and rises. As soon as it reaches the pond’s surface, is loses its heat to the air as water vapour or by convection currents. The coolest water, which is considered the densest and heaviest, changes places with warm water which has risen to the surface, thus creating a natural carrying movement which mixes up the water and disperses the heat energy.

Solar Pond in the Dead Sea

In order to extract heat from the water of the Dead Sea, a small, square Solar Pond, 1.25 metres deep and 2.0 metres wide was designed as a test by Hashem al-Balawneh, an engineering student from Jordan, under the guidance of Dr. Khaldun al-Wahoosh. This solar pond was constructed in the Dead Sea region, at the coordinates 0 20 30 N, 0 30 35 E. Heat is prevented from escaping via convection by the Dead Sea water’s specific salinity, as well as by the addition of a group of Sodium Chloride, Magnesium Chloride and Sodium Bicarbonate salts (NaCl, MgCl₂ and NaHCO₃), which are also extracted from the Dead Sea.

Solar Ponds in the Dead Sea have a certain characteristic which allows them to keep heat energy, and that is the increase in salinity with increased depth. Accordingly, density also increases with depth, forcing the warm water to stay lower down because of the salts. Next, the heat which the water has absorbed in the last, salt-saturated layer whose temperature can reach between 85-90°C – moves turbines, thus generating clean, renewable, environmentally-friendly electrical energy.

Importance of Solar Ponds

Solar Ponds provide the simplest technique for transforming the sun’s energy into solar power, which can be extracted for different purposes. Solar Ponds are unique in their ability to gather and store energy simultaneously. It is known that the cost of Solar Ponds per unit area are less than any other current popular solar energy collector, as well as the fact that the continuous fluctuations in oil prices in recent times have pushed many individuals and organisations to look for other, cheaper, renewable sources of energy.

Similarly, the warm water which we get after extracting the pond’s heat can then be put to multiple industrial uses and to heating greenhouses in or around the Dead Sea region when the winter frosts set in. Solar Ponds can be used in all climates, as long as there is lots of sun, and even if the pond froze over, it would still be able to generate energy as it is saturated with salts. For an efficient, energy-generating Solar Pond to be set up, the following are needed: a relatively large area of low-cost land, water with high salinity and lots of sunshine. All these prerequisites are abundant in the Dead Sea region, which is the lowest and saltiest body of water in the world. Solar Pond system in the Dead Sea will help in large-scale energy storage and should be seen as an innovative step in the field of energy production and development in Jordan.

 

Translated by Katie Holland

Katie Holland graduated from Durham University in 2015 with a degree in Arabic and French, having also studied Persian. Currently working in London, she hopes to develop a career that uses her knowledge of Arabic and the Middle East, alongside pursuing her various interests in the arts. 

Biomass Energy and its Promise

Biomass is the material derived from plants that use sunlight to grow which include plant and animal material such as wood from forests, material left over from agricultural and forestry processes, and organic industrial, human and animal wastes. Biomass comes from a variety of sources including wood from natural forests, agricultural residues, agro-industrial wastes, animal manure, organic industrial wastes, municipal solid wastes, sewage sludge etc. When biomass is left lying around on the ground it breaks down over a long period of time, releasing carbon dioxide and its store of energy slowly. By burning biomass its store of energy is released quickly and often in a useful way.

Technology Options

Biomass resources can be transformed into clean energy and/or fuels by a variety of technologies, including thermal and biochemical. Besides recovery of energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal.

As far as thermal technologies are concerned, biomass can be converted into energy by simple combustion, by co-firing with other fuels or through some intermediate process such as gasification and pyrolysis. The energy produced can be high calorific value gases, electrical power, heat or both (combined heat and power). The advantage of utilizing heat as well as or instead of electrical power is the marked improvement of conversion efficiency – electrical generation has a typical efficiency of around 30%, but if heat is used efficiencies can rise to more than 85%.

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat. In addition, biomass can also yield liquid fuels, such as bioethanol or biodiesel, which can be used to replace petroleum-based fuels. Algal biomass is also emerging as a good source of energy because it can serve as natural source of oil, which conventional refineries can transform into jet fuel or diesel fuel.

Applicability

Biomass energy technology is quite flexible and can be applied at a small, localized scale primarily for heat, or it can be used in much larger base-load power generation capacity whilst also producing heat. Biomass generation can thus be tailored to rural or urban environments, and utilized in domestic, commercial or industrial applications.

Biomass conversion reduces greenhouse gas emissions in two ways.  Heat and electrical energy is generated which reduces the dependence on power plants based on fossil fuels. GHG emissions are significantly reduced by preventing methane emissions from landfills.  Moreover, biomass energy plants are highly efficient in harnessing the untapped sources of energy from biomass wastes.

Major Benefits

Biomass energy systems offer significant possibilities for reducing greenhouse gas emissions due to their immense potential to replace fossil fuels in energy production. Biomass reduces emissions and enhances carbon sequestration since short-rotation crops or forests established on abandoned agricultural land accumulate carbon in the soil.

Biomass energy usually provides an irreversible mitigation effect by reducing carbon dioxide at source, but it may emit more carbon per unit of energy than fossil fuels unless biomass fuels are produced unsustainably. Biomass can play a major role in reducing the reliance on fossil fuels. In addition, the increased utilization of biomass-based fuels will be instrumental in safeguarding the environment, generation of new job opportunities, sustainable development and health improvements in rural areas. Biomass energy could also aid in modernizing the agricultural economy.

When compared with wind and solar energy, biomass plants are able to provide crucial, reliable baseload generation. Biomass plants provide fuel diversity, which protects communities from volatile fossil fuels. Since biomass energy uses domestically-produced fuels, biomass power greatly reduces our dependence on foreign energy sources and increases national energy security.

Global Trends

Biomass energy has rapidly become a vital part of the global renewable energy mix and account for an ever-growing share of electric capacity added worldwide. As per a recent UNEP report, total renewable power capacity worldwide exceeded 1,470 GW in 2012, up 8.5% from 2011. Renewable energy supplies around one-fifth of the final energy consumption worldwide, counting traditional biomass, large hydropower, and “new” renewables (small hydro, modern biomass, wind, solar, geothermal, and biofuels).

Traditional biomass, primarily for cooking and heating, represents about 13 percent and is growing slowly or even declining in some regions as biomass is used more efficiently or replaced by more modern energy forms. Some of the recent predictions suggest that biomass energy is likely to make up one third of the total world energy mix by 2050. Infact, biofuel provides around 3% of the world’s fuel for transport.

Introduction to Trigeneration

Trigeneration refers to the simultaneous generation of electricity and useful heating and cooling from the combustion of a biomass fuel or a solar heat collector. Conventional coal or nuclear-powered power stations convert only about 33% of their input heat to electricity. The remaining 67% emerges from the turbines as low-grade waste heat with no significant local uses so it is usually rejected to the environment.

What is Trigeneration

In a trigeneration system, the supply of high-temperature heat first drives a gas or steam turbine powered generator and the resulting low-temperature waste heat is then used for water or space heating. Such systems can attain higher overall efficiencies than cogeneration or traditional power plants, and provide significant financial and environmental benefits.

Trigeneration is one step ahead of cogeneration that is the residual heat available from a cogeneration system is further utilized to operate a vapor absorption refrigeration system to produce cooling; the resulting device thus facilitates combined heat power and cooling from a single fuel input. The heat produced by cogeneration can be delivered through various mediums, including warm water (e.g., for space heating and hot water systems), steam or hot air (e.g., for commercial and industrial uses). 

Advantages of Trigeneration

Trigeneration is an attractive option in situations where all three needs exist, such as in production processes with cooling requirements. Trigeneration has its greatest benefits when scaled to fit buildings or complexes of buildings where electricity, heating and cooling are perpetually needed. Such installations include but are not limited to: data centers, manufacturing facilities, universities, hospitals, military complexes and colleges. Localized trigeneration has addition benefits as described by distributed generation. Redundancy of power in mission critical applications, lower power usage costs and the ability to sell electrical power back to the local utility are a few of the major benefits.

Most industrial countries generate the majority of their electrical power needs in large centralized facilities with capacity for large electrical power output. These plants have excellent economies of scale, but usually transmit electricity long distances resulting in sizable losses, negatively affect the environment.

Large power plants can use cogeneration or trigeneration systems only when sufficient need exists in immediate geographic vicinity for an industrial complex, additional power plant or a city. An example of cogeneration with trigeneration applications in a major city is the New York City steam system. The city of Sydney has embarked upon an ambitious trigeneration plan to reduce greenhouse gas emissions by 70 percent by producing 477 MW of local power using trigeneration systems.

One of the technologies that have the best performance for being integrated into a trigeneration system is the fuel cell. Systems working on fuel cell technology can transform the energy of a chemical reaction into electrical energy, heat and water. Its main practical applications range from bulk production of electricity and heat to its use in sectors such as aerospace, maritime or surface transport and portable devices.

Trigeneration Prospects in the Middle East

There is very good potential for deployment of trigeneration in the Middle East. The constant year-round heat coupled with expensive glass exteriors for hotel, airports, offices, apartments etc result in very high indoor temperatures. The combination of distributed generation of power and utilization of waste heat can provide a sustainable solution to meet the high demand for refrigeration in the region. District cooling has the potential to provide a viable solution to meet air conditioning requirements in commercial buildings, hotels, apartment blocks, shopping malls etc.

Trigeneration systems can play a vital role in reducing energy requirements in Middle East nations. Apart from providing cooling needs, such systems can reduce the need for new power plants, slash fossil fuel requirements and substantially reduce greenhouse gas emissions from the region.   

UHI Effect: Impact on Sustainability

Urban Heat Island (UHI) Effect arises due to absorption of incident radiation from the sun by built surfaces of tall buildings, roof, concrete structures and asphalt roads and then releasing it in the form of heat. The term “urban heat island” describes the built-up areas that are significantly hotter than the surrounding open, natural or rural areas. It occurs on the surface and in the atmosphere. The built surfaces are made of high-percentage of non-reflective and water-resistant construction materials. These materials act as heat sinks that absorb the radiated heat and store it for long time.

The UHI Phenomenon

Lack of sufficient wind, change in thermal properties of the surface materials and lack of evapotranspiration rate in urban areas cause the urban heat island effect. On the other hand, green, wooded and open spaces composed of vegetation and moisture trapping soil use large proportion of absorbed radiation and release them through evapotranspiration process. As evaporation causes cooling effect, the released water vapour contributes to cool the air in the vicinity. On a hot summer day, the urban surfaces are exposed to high temperature of   50–90°F (27–50°C) hotter than the air, where as the temperature of the shades or green open areas surrounding the urban surfaces remain close to air temperature. These changes in temperature between two areas create an “island” of higher temperature in the urban landscape. Normally the temperature difference of higher than 10 degrees forms heat islands.

Impacts on Sustainability

The increase in temperature in urban areas due to UHI effect can have negative impacts on three pillars of sustainability, i.e. environment, people and economy. Some of the negative impacts include:

  • Increase in energy consumption – Increase in temperature leads to increase in demand for cooling, which subsequently puts pressure on electricity supply during the peak periods of demand.
  • Increase in emission of air pollutants and GHGs – As more electricity is needed to cool the surfaces, demand on energy supply leads to emissions of air pollutants and greenhouse gases from the power plants. Even use of ozone depleting refrigerants such as CFCs in the air-conditioning system cause depletion in stratospheric ozone layer. Elevated temperature also promotes the formation of ground-level ozone.
  • Demand on water – As the surface and air get hotter, people consume more water for both indoor and outdoor usage and it puts pressure on water supply.
  • Ecosystem – Hot surfaces transfer the absorbed heat to water features such as rivers, streams, ponds, lake etc. increase the surface water temperature and alerting the aquatic  ecosystem structure and functions
  • Quality of life – Elevated day and night temperatures along with higher air pollution can cause respiratory diseases, discomfort, heat stress and decrease productivity and increase heat related mortalities.

UHI Effect in the UAE

UHI is quite common in cities located in the temperate zone. However, a very few studies are done so far to find how cities in semiarid and arid areas act as urban heat islands. UAE consists of seven emirates and weather here is tropical desert climate. Out of seven emirates, Dubai, Abu Dhabi and Sharjah have experienced a rapid rise of high and low intensity urban areas in recent years.

Dubai the most populated and developed emirate and a very few studies indicated that its urban climate is mostly affected by land use changes, vegetation cover, and expansion of built of areas. It was thought that cities in arid region have possibility to act as daily urban cool islands (UCI). However, there are not many studies done so far to establish this. Rather some studies indicated that Dubai has seen 64.8% change in land cover and a 1.5 degree C rise in land surface temperature (LST) in past 10 years. These are the common indicators of UHI.

Mitigation Measures

Studies have found that the mean daily temperature increase is consistent with increase in urban development. The composition of land cover features can significantly influence the magnitude of land surface temperature.  Hence, increase in percent of vegetation is the most essential driver of reducing the land surface temperature and hence the UHI effect. Therefore, proper management of green space is needed to mitigate the UHI effect in the urban cities of arid and semi arid countries. The heat island effect can be reduced by using following strategies.

  • Build small – Minimise building footprint and maximise open space
  • Minimize hardscape – Design driveways, roads, parking space and hardscape areas smartly by using permeable materials or surfaces such as vegetated roofs, porous pavement and grid pavers. Use open grid pavement system, which is at least 50% pervious and locating the parking space under the building will help reducing the urban heat island effect.
  • Use of reflective materials – Use high reflective materials with high solar reflective index (SRI) values for roofs and non-roof exterior surfaces.  The SRI value is the combined value of reflectivity and emmitance.
  • Shading – Provide shading with existing tree canopy or new trees or with other structures. The surfaces can also be coved by solar panels that produce renewable energy. Shading with some architectural features of SRI of at least 29 will also help to reduce the heat island effect.
  • High albedo cool roof and green roofs: Combination of high albedo cool roofs (roofs with controlled SRI) and vegetated roof surface can reduce heat island effect significantly.

Conclusions

The composition of land cover features can significantly influence the magnitude of land surface temperature.  Hence, increase in percent of vegetation is the most essential driver of reducing the land surface temperature and hence the UHI effect. Therefore, proper management of green space is needed to mitigate the UHI effect in the urban cities of arid and semi arid countries.