By Michael Welch and Andrew Pym
For the past 100 years across most of the world, consumers have received their electricity from large central power plants, which provide energy to the entire system from a single location via a network of transmission lines. This model, which relies heavily on fossil fuels, is facing an increasing number of challenges.
The major initial efforts to reduce the environmental impact of power generation centered on fuel switching from coal to natural gas, with plans for massive centralized coal-fired power stations giving way to more efficient, less polluting, natural gas-fired power plants in the so-called “dash for gas,” changing the power mix from predominantly thermal coal-fired steam turbine plant to a more even split between coal and combined cycle gas turbines.
|A 6 MWe ORC installation with air-coled condenser in Germany. Photo courtesy: Siemens|
With increasing global efforts to reduce greenhouse gas emissions, there is an increasing penetration of intermittent and variable renewable energy. Both wind and solar generation output vary significantly over the course of hours to days, sometimes in a predictable fashion, but often imperfectly forecast. This intermittency and variability of wind and solar power generation presents challenges for grid operators to maintain stable and reliable grid operation, especially in countries where renewable power is given dispatch priority, requiring redundancy and flexibility in fossil-fueled power generation so that the system can respond quickly to these fluctuations, outages and grid support obligations. Predominantly to date this has been achieved by operating central power plant so that they maintain their connection to the grid but run at part-load so that they can rapidly respond to transients on the system network.
Without sufficient system flexibility, system operators may need to curtail power generation from wind and solar sources. The centralized power generation model has created a trend over the past century towards ever increasing unit sizes, based on the assumption that larger units and bigger plant provided lower cost power generation due to economies of scale, with small increases in power generation efficiency also contributing to this. The accepted penalty was losses in the transmission and distribution networks, and the potential for consumers to lose their power supply in case of transmission or distribution system outages. However, maximum efficiency occurs at full-load, so operating a large central plant at part-load reduces the efficiency of power generation considerably, and the need for part-load operation may impact on the operational range of the power station due to the need to comply with emissions legislation. In addition, cycling of the units, ramping up and down in load, can create the need for more frequent maintenance and power station outages. A large utility-scale turbine undergoing major maintenance can require around two to three weeks outage for disassembly, inspection, parts replacement and reassembly. Cycling also reduces part life and severely impacts plant economic returns and in some cases, overall viability.
Another issue facing centralized power generation is water usage. In many parts of the world, water is a scarce resource for which power generation competes with agricultural, industrial and domestic needs. In 2010, World Bank estimates indicated 15 percent of the world’s water withdrawals were used for energy production, and with electricity demand expected to grow 35 percent by 2035, water usage for power generation will increase significantly, especially in systems relying on the centralized generation model.
Distributed Generation can help address all the above issues. By building smaller, more flexible power plants closer to the actual load centers, network operators can better compensate for the intermittency of renewables, reduce transmission system losses and improve security of supply and reduce capital expenditure on capacity expansion/augmentation while the power plant operators by using multiple units can optimize the plant design to meet the needs of the network operators with fast ramp up and turn down and the ability to operate at low output levels, while still maintaining high efficiencies, low emissions and low power plant maintenance downtimes. Distributed Generation is also enabler for enhanced smart grid capabilities.
Flexibility of a Multiple Gas Turbine Solution
Conventional modern large-scale Combined Cycle Gas Turbine power plant (CCGT) are usually based on a single gas turbine with a single steam turbine (1+1 configuration), or two gas turbines with a common steam turbine (2+1 configuration). While this configuration offers very high efficiencies at full load, in excess of 60 percent today, the efficiency falls as load reduces. There is also a minimum emissions compliance load, which limits the operating range of the power plant.
With around 1/3 of the total station power generated by the steam turbine, it can take over 30 minutes to achieve full station load. In addition, with the gas turbine shut down for maintenance in a 1+1 configuration, the complete station is offline, whereas in a 2+1 configuration, an outage of one gas turbine will reduce station power generation to less than 50 percent of its rated output. A solution based on multiple gas turbines may offer much greater flexibility, improved efficiency across the power range and enhanced operability compared to a conventional CCGT solution.
The Advantages of Modularity
Modularity can help enhance plant flexibility and reliability. By having multiple units, load can be shared across them, and units switched on and off to match the required load. This enables the power plant to operate efficiently over a much wider load range within the permitted emissions limits than a conventional CCGT can achieve. Future plant expansion is easy to achieve simply by adding one or more units whenever required, either at the same location or at a different tactical point in the power network, rather than having to build a new large power plant and associated transmission system. By distributing capacity in this way a ‘virtual generation’ benefit is also achieved via loss offset in the transmission network. The modular attributes also enable plant to be moved easily if market conditions change or the plant is sold. This reduces operational and financial risk which is beneficial for accessing finance at more favorable terms. Small gas turbines tend to come in pre-designed, pre-assembled standardized packages which have undergone significant levels of factory testing and require only a simple concrete foundation. This reduces the amount of planning, engineering, site installation and construction work required compared to a conventional power plant, enabling the power plant to be brought online faster, while still maintaining a competitive first cost, and reduces the risk of construction delays and associated contract penalties in addition to lost revenue. In addition, these packages can be supplied with weather-proof acoustic enclosures, eliminating the need for buildings. All the auxiliary systems required for turbine operation – including the control system – can be mounted either within the enclosure, adjacent to the enclosure or on the enclosure roof, minimizing the number of interconnections required.
Having multiple units also helps maintain high power plant availability and output. As mentioned earlier, with a single gas turbine installation, a maintenance outage means that the entire power station has to be taken offline. A power plant of similar output but based on, say, 5 smaller gas turbines can still generate 80 percent of rated station output with one turbine out of service, 60 percent with two turbines out etc. Decentralized power plant using this concept have been used for many years in the Oil & Gas industry for onshore fields and offshore platforms with no possibility to connect to a power grid, with many Oil & Gas operators choosing the so-called ‘N+1’ configuration so that there is a spare unit to ensure 100 percent power output is available even with one gas turbine out of service.
The ability of a power plant to respond rapidly to variable grid demands is critical in today’s power environment with a high percentage of intermittent renewable power generation. Multiple small gas turbines allow the full plant load to be achieved relatively quickly from pushing the start button as the units can ramp up in parallel.
The ramp rates of small gas turbines typically range between 100 kW/second and 200 kW/second.
However, gas turbines can also accept step load applications while still maintaining power generation within the required frequency and voltage limits. The maximum acceptable step load depends on the gas turbine design – a single shaft gas turbine can accept a larger single load application than a twin-shaft variant – but this ability to step load enables the turbines to reach full load much faster than by employing a simple ramp rate for loading. Figure 4 shows the comparison of time taken for a twin-shaft 12MW gas turbine to reach full load using the maximum permissible load steps for this particular gas turbine model – full load can be achieved in half the time by applying load in steps.
Single-shaft gas turbine designs can accept greater step loads, varying from 50 percent to 100 percent depending on the model, rating and site conditions. In the case of a 50MW single-shaft gas turbine, it is possible to load the unit from zero to full load in two steps within 30 seconds.
Reducing Maintenance Outages
When scheduled maintenance is required and parts need to be replaced, the large utility scale gas turbines require considerable downtime as the unit has to be disassembled on site, parts changed and then the unit reassembled. The smaller gas turbines are generally of Light Industrial or Aeroderivative designs which, while many variants have the capability for on-site maintenance as well, are primarily designed for off-site maintenance employing gas generator and turbine module exchange programs. This reduces the turbine outage times for major inspections from several weeks per unit to between one day and five days depending on the gas turbine model and the type of maintenance intervention required. Meanwhile in a power plant based on multiple units, the remaining units are still available to generate power, enabling the power station to stay online generating revenue, with only a relatively small percentage of total plant output unavailable.
Routine maintenance requirements during plant operation are also low, with no requirement for highly skilled maintenance personnel to be permanently based on site and low consumption of consumables such as lubricating oil. The various gas turbine OEMs are all working on further developments to improve system reliability and remote monitoring systems to enable unmanned operation for prolonged periods of time.
As has been well-documented elsewhere, the output of a gas turbine is dependent on ambient temperature: as ambient air temperature rises, a gas turbine’s power output reduces. Conversely this means that if you design a power plant to give a specific output at the maximum ambient temperature foreseen, on cooler days more power is available for dispatch. If there are distribution or transmission system constraints that limit the amount of power that can be exported, then on cooler days, while still producing maximum station output, the gas turbines will operate at part-load. Most GT OEMs calculate the time between overhaul (TBO) for the various different gas turbine models based on an Equivalent Operating Hours (EOH) formula – part-load operation can help extend the TBO reducing the maintenance requirements still further.
While Utility-scale gas turbines are designed primarily for operation on pipeline quality natural gas with a premium liquid fuel such as diesel as an alternative or back-up fuel, the majority of smaller gas turbine models are able to operate on a much wider range of gaseous and liquid fuels.
Low emissions combustion systems have also been developed that will operate on non-standard gas fuels, including those with variable compositions. This is a potentially important feature for decentralized power plant as it enables the power plant to operate on a locally available fuel, which, as some of these are classified as waste gases, may also be more economical than utilizing pipeline quality natural gas. Examples of such potential gas fuels are landfill gas, digester gas, high hydrogen content gases such as refinery gas or syngas, ethane and propane. It is potentially possible to use two completely different gas fuels and switch between these fuels as necessary, determined by fuel availability or pricing.
Most gas turbines are available in dual fuel configuration, able to operate on either gas fuel or liquid fuel. The turbines can operate on 100 percent gas fuel or 100 percent liquid fuel, with rapid automatic changeover between the fuels with no requirement to temporarily reduce load to undertake the fuel change. The liquid fuels that may be considered are typically #2 diesel, kerosene, LPG and naphtha, although there are gas turbine models available that can utilize Light, Intermediate and Heavy Fuel Oils, Residual Oils, Bio-Oils and even Heavy Crude Oils. On some gas turbines it is possible to simultaneously operate on both gas and liquid fuels – commonly referred to as bifueling or mixed fuel operation – using one fuel type to compensate for shortage of another.
There are examples of tri-fuel gas turbine installations, with units capable of operating on a gas fuel and two different liquid fuels, or a liquid fuel and two different gas fuels. Figure 5 is a gas turbine installed in a cogeneration plant at a university in the U.S. and configured to operate on either pipeline quality natural gas or a processed landfill gas, with diesel as a back-up fuel in case of loss of gas supplies, while still meeting strict emissions limits.
Improving Part-Load Efficiency and Emissions Performance
Smaller open cycle (simple cycle) gas turbines have been used for peaking applications for many years because they can be started quickly and ramped up and down rapidly to meet the grid demands. In open cycle, a gas turbine is relatively inefficient with efficiencies varying from around 28% for a small industrial gas turbine to just over 40 percent for the larger aero-derivative gas turbines. In peaking applications, this is perhaps not so much of an issue as the price of electricity is very high during the periods of gas turbine operation, but with increasing demand for flexible power generation across the whole day, a power plant today needs to be able to operate efficiently and in an environmentally friendly manner for base load, load following and peaking service. Figure 6 compares the net plant efficiency of a single 50MW class aero-derivative gas turbine in open cycle with four open cycle 12.5 MW class gas turbines with performance data calculated for an ambient air temperature of 40° C. While at high loads the single unit is more efficient, once the power plant output drops below 50% of rated plant output, the multiple unit solution has a higher efficiency as units can be turned on and off to maximize efficiency. The multiple unit solution also offers a wider power plant operating range from a combustion emissions perspective. Most gas turbine models guarantee nitrous oxide (NOx) and carbon monoxide (CO) from 50% of rated load to 100% of rated load, as required by most global legislation, although some units offer these guarantees down to 30% or 40% load. Therefore a single unit solution at low loads will start to exceed the permitted emissions.
A multiple unit solution though enables the power plant to have a greater turn-down capability while still complying with applicable emissions legislation. In the example in Figure 6, and assuming 50 percent turndown limit, the power plant will still meet emissions requirements down to 12.5 percent of rated power plant output. However, for a truly flexible power plant, the efficiency of the gas turbines needs to be as high as possible as well as providing as wide an operating range for the power plant as possible. While there are complex cycle gas turbines on the market with recuperators and intercooling to improve efficiency, the simplest, most effective and most proven way to improve efficiency is to use a combined cycle configuration with energy recovered from the exhaust of the gas turbine to generate additional power. Water (steam) is the obvious choice as a working fluid to generate additional power via a steam turbine, just as in a conventional large-scale CCGT. However, smaller gas turbines are not optimised for combined cycle applications, having relatively low exhaust mass flows and exhaust gas temperatures, and although combined cycle efficiencies in excess of 55 percent can be achieved, the complexity of the high pressure, high temperature heat recovery steam generators (HRSGs) and steam turbine systems required to achieve this efficiency level adds considerable cost. Lower cost solutions using low pressure steam systems can be employed, but this reduces the plant efficiency. In addition, for decentralized plant located close to load demand, the availability of water may be an issue, or the operation and maintenance level required by classical steam solution cannot be easily accommodated, so an alternative technology to generate electricity from the wasted energy in the gas turbine exhaust needs to be considered.
Organic Rankine Cycle (ORC) Technology
The Rankine Cycle is a thermodynamic cycle which converts heat into work. For power generation, by applying heat externally to a closed loop, the working fluid is heated till it becomes a vapor, expands across a turbine to drive a generator and is then cooled and condensed ready to commence the cycle again. Water is normally the working fluid used, and the water (steam)-based Rankine Cycle provides approximately 85 percent of worldwide power generation.
A utility scale gas turbine tends to have a high exhaust gas temperature, typically between 530°C (990°F) and 640°C (1180°F), as the designs are optimized for combined cycle applications with multi-pressure level multi-pass boilers producing high pressure, superheated steam (up to 160 bar and 600°C) for inlet to steam turbines with reheat between different pressure levels within the steam turbine. This is how a modern CCGT achieves the high full load efficiencies quoted, and produces electricity at competitive prices through economies of scale.
Smaller gas turbines have lower exhaust gas temperatures, typically between 460°C (870°F) and 550°C (1025°F) as they are optimized for maximum open cycle efficiency. This reduces both the volume and temperature of high pressure superheated steam that can be produced, reducing cycle efficiency. It is also not cost-effective to use the same Waste Heat Recovery Unit and Steam Turbine technology as developed to go with a 300 MW gas turbine on a 10 MW gas turbine.
Therefore if a power plant is to be based on multiple small units, efficiency must be sacrificed to ensure cost-effectiveness, so lower pressure non-reheat steam systems are used, often with much simpler Once Through Steam Generators (OTSGs) that respond much more rapidly to changes in steam demand. However, at low pressures, there is a large enthalpy drop experienced when water is the working fluid, and a degree of superheat is required to avoid the risk of condensation, and associated erosion, inside the steam turbine.
By changing the working fluid, a low enthalpy drop can be achieved, the need for superheating eliminated, as condensation within the turbine can be avoided, and the same efficiency achieved at a lower working pressure. Improved efficiencies at part-load are also attainable using ORC turbogenerators compared to conventional steam turbines.
Organic Rankine Cycles for small gas turbines tend to use a high molecular weight hydrocarbon (organic) fluid such as cyclopentane, or silicone oil, as the working fluid for the turbine. This allows high efficiency, larger diameter turbines to be utilized, operating at lower speeds, typically 3000rpm, with low mechanical stress – unlike small steam turbines which operate at speeds up to around 10000rpm. The combination of working fluid and turbine speed leads to much reduced maintenance requirements, as well as eliminating the need for water in the process.
ORC systems can use either directly or indirectly heat the working fluid. In both cases the Waste Heat Recovery Unit (WHRU) installed in the gas turbine exhaust system is a simple once through design, but in the indirectly heated system, heat is transferred from the gas turbine exhaust to the ORC working fluid via a secondary closed loop using a thermal oil. Directly heated systems offer better efficiency of the ORC cycle (see Figure 10 below) and reduce the initial capital cost, while an indirectly heated system allows for energy to be recovered from higher temperature heat sources than a directly heated system.
Combining Gas Turbines + ORC to Maximize Performance
As mentioned earlier, smaller gas turbines are not optimised for combined cycle applications, generally having lower exhaust temperatures than the utility scale gas turbines, and so they have reduced high pressure steam raising capabilities. However, the lower exhaust temperatures at both full and part-load enable ORC technology to be readily employed to improve overall plant efficiency while still enabling multiple units to be installed to maintain the overall power station flexibility and operability. This configuration also has the additional advantage of being able to be ‘water free’ as air cooling can be used throughout the installation.
Returning to our power plant capable of producing 40 MW at 40°C referred to in Figure 6, the addition of an ORC turbogenerator to the smaller gas turbines has quite a considerable impact, as can be seen in Figure 12.
Firstly, it can be seen that the ORC system adds about 25% additional power output for the multiple small units for no additional fuel input. Secondly, this additional power improves the plant efficiency so that at full load the overall net plant efficiency is in excess of 40%, even on a hot day. This efficiency improvement makes a nominal 50MW plant based on multiple gas turbines more efficient and more flexible than a plant based on a single open cycle gas turbine across the whole load range, and with the ability to achieve load turn-down to around 10 percent of rated station power output while still maintaining an acceptable combustion emissions profile.
Multiple gas turbines can be connected to a single ORC turbogenerator, providing the maximum output rating of the ORC turbogenerator is not exceeded. This helps reduce the cost/kW of a power plant based on multiple gas turbines as the cost of the ORC system is spread across multiple units. In addition, thanks to ORC working fluid peculiarities, the plant flexibility and efficiency at part load is not reduced. The ORC unit can be operated at between 10 percent and 110 percent of its nominal load automatically, while still maintaining high efficiency even at partial load – as shown in the Figure 13, at 50 percent of the load, the ORC still has an efficiency of 90 percent of nominal full load efficiency).
Obviously it is also possible to add an ORC system onto the larger gas turbine considered to improve efficiency but it is most likely in these cases that a configuration based on each gas turbine having its own ORC turbogenerator system will be needed.
As for the multiple small units, the addition of the ORC system boosts both power and efficiency considerably. At nearly 47 percent net power output on a 40°C ambient day with all site losses accounted for, a 50MW class gas turbine with ORC offers a better efficiency than large (100MW) complex cycle gas turbines (which are quoted as having an ISO, zero loss efficiency of 44%). It is interesting to note that while the efficiency of the single larger gas turbine plus ORC is higher for station loads over 60 percent, at lower loads the efficiency of the multiple small units plus ORC is better. This suggests that for larger power plant of, say, 200 MW or 250 MW design output, a combination of 50 MW class and smaller 12.5 MW class gas turbines would give the optimum plant efficiency across the widest load range. The gas turbine plus ORC combination helps maintain a high output power and high net plant efficiency across a wide temperature range. In the example given in Figure 14, the ORC system is air-cooled, with the Air Cooled Condenser (ACC) designed for an average 30° C ambient temperature. The shape of the power output and efficiency curve can be altered by the design temperature used for the ACC: designing the ACC for the maximum ambient temperature will impact plant performance at lower temperatures, so it is important to consider and define the correct design point.
Combining multiple small gas turbines with ORC technology permits engineers to design a very load flexible power plant with optimal efficiency and emissions compliance across a wide load range. With no requirement for a water supply, such modular power plant potentially offer a simple way to meet the demands on the electricity grid caused by the large amounts of intermittent renewable power generation with high power plant reliability, availability and low maintenance in a cost-effective manner. By building such flexible distributed plant close to the actual load centers, investment in the power system infrastructure can also be reduced.