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.

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 the 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.

District Cooling Perspectives for the Middle East

District cooling produces chilled water in a centralized location for distribution to buildings like offices and factories through a network of insulated underground pipes. The chilled water travels to different buildings, where the water circulates through refrigeration coils or uses absorption technology to enter the air-conditioning system. During winter, the source for the cooling can often be sea water, so it is a cheaper resource than using electricity to run compressors for cooling.

What is District Cooling

District cooling provides effective control over internal temperature of a building, requires less maintenance than a standalone air-conditioning system, consumes lesser space and reduces noise pollution. The effect of district cooling systems on the environment is minimal because of the reduction in carbon dioxide emissions, use of eco-friendly refrigerants and implementation of rigorous health and safety standards.

The Helsinki district cooling system in Finland uses waste heat from CHP power generation units to run absorption refrigerators for cooling during summer time, greatly reducing electricity usage. In winter time, cooling is achieved more directly using sea water. The adoption of district cooling is estimated to reduce the consumption of electricity for cooling purposes by as much as 90 percent. The idea is now being adopted in other Finnish cities.

The use of district cooling is also growing rapidly in Sweden and in a similar way. District cooling is very widespread in Stockholm, the capital of Sweden. In fact, approx. 7 million square meters of commercial space in Stockholm is connected to the district cooling grid. The Stockholm district cooling grid currently consists of different systems with capacities ranging from 3 MW to 228 MW. The district cooling network in Stockholm is currently 76 kilometers long.

District Cooling Prospects in the Middle East

There is tremendous potential for the utilization of district cooling systems in the Middle East. The constant year-round heat coupled with expensive glass exteriors for hotel, airports and offices 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 systems can provide cooling solutions to commercial buildings, hotels, apartment blocks, shopping malls etc.

The world’s largest district cooling plant, Integrated District Cooling Plant (IDCP), was installed in The Pearl-Qatar in 2010. IDCP will service more than 80 apartment towers, beachfront villas, townhouses, shopping complexes, offices, schools and hotels throughout the Island, ultimately supplying more than 130,000 tons of refrigeration to the Island’s estimated 50,000 residents.

Despite paramount importance of air conditioning in Middle East countries, regional governments have failed to incorporate it in policy and planning which has lead to the evolution of an unregulated market for cooling systems.  Most of the cooling methods employed nowadays are based on traditional window units or central air cooling systems where consume copious amount of power and also damage the environment.

District cooling has the potential to provide a viable solution to meet air conditioning requirements in the Middle East. Low energy requirement, peak saving potential, eco-friendliness and cost-effectiveness are major hallmarks of district cooling networks. District cooling can play a vital role in fostering sustainable development in Middle East nations. Apart from providing cooling needs, district cooling can reduce the need for new power plants, slash fossil fuel requirements and substantially reduce greenhouse gas emissions from the region.   

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.