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Gas-engines in tandem with renewables

21st November 2017


MTU Onsite Energy gensets are ideal for use in hybrid power plants
Modular development: this configuration features five gensets along with combined-cycle steam turbines

The rapid scale-up of many emerging power markets, at the same time as an introduction of higher levels of solar, hydro and wind renewables, can lead to a mismatch between local power needs and the capacity of the network to meet those needs. Simultaneously, there has been a rapid expansion in the availability and extraction of natural gas, although many areas lack gas distribution network infrastructure.

Whether you are powering a single factory or an entire community, you need power you can trust. A quick recovery is also needed when local power consumption has unexpectedly overtaken supply and created high peak demands that threaten the stability of the national or regional grid.

Rolls-Royce Power Systems (RRPS) can provide an effective means to erect power stations based on reciprocating gas engines in more remote areas rich in gas, while producing significant levels of electrical power to support the national electricity need. In addition, the rapid start-up, quick shutdown and fast ramp rates allow for the quick response needed to meet the variable generation of non-hydro renewables in microgrids, while the use of multiple units enables high levels of plant turn down and availability throughout the year. The benefits become even clearer when seen within the framework of a modular power system where the use of pre-defined customisable modules allows specific site needs to be accommodated, and to keep down the overall plant lead time from order to commercial operation date.

Gas is used as significant contributor to clean power and clean industrial production. In the context of a global forecast demand, the share of natural gas in the global energy mix increases from 19% in 2014 to 23% in 2040, drawing level with coal as the second largest fuel in the global energy mix after oil. The main regions pushing global gas demand higher are China, which will become a larger gas consumer than the EU by 2035, and India. Gas plays an important role in mitigating coal use and related air pollution in these regions.

The regional share of gas demand taken by power generation is expected to remain stable at around 37%. However, the gas share taken directly by industry will increase from 21% to almost 30%. This increase in share from industry, coupled with overall increased demand for gas and the large future regional deficit, presents a major challenge for economic development. It is a clear goal stated by many governments to reduce the carbon intensity of their expanding economic activity and to increase the geographical diversity of that economic activity. The addition of grid-scale variable renewable energy (RE) to the network supports the first goal. Unfortunately the location of suitable sites for variable RE and the desirable sites for industrial expansion often do not coincide. If the electricity networks can be improved, it is practical to place the needed balancing power stations close by the new centres of electrical demand. In these cases the required gas pipelines can serve both the industrial and power generation need.

The regional gas deficit and increased demand requires liquefied natural gas (LNG) importation terminals to be constructed and a gas distribution infrastructure to meet the needs of new industrial areas. Gas-based power can operate at the convenient nexus between both of these developments, balancing the inherent instability of variable RE through the new electrical networks forming so called microgrids and drawing upon the new gas infrastructure being built for industrial growth.

Comparison of technologies
Power developers of distributed generating systems as add-on to renewable energy sources have a choice between two primary power sources: gas reciprocating engines and gas turbines. Both are proven worldwide in thousands of pure power (simple cycle) and cogeneration (combined heat and power) installations. Over the years, both technologies have steadily improved in efficiency, reliability, emissions performance and operating costs – and they continue to do so.

A most effective use of gas in power generation is as part of a reciprocating engine or a combined-cycle gas turbine (CCGT) solution. As the reliable availability of gas has improved, the continuous power reciprocating engine market has shifted towards pure gas engines. In 2014, for the first time, the market share of pure gas reciprocating engines, in the largely continuous power market of >3.5MW units, exceeded the combined share of HFO and dual-fuel engines.

Many papers have been presented demonstrating the advantages of large reciprocating engine-based power and in an era of de-regulation and increase in the number of IPP projects, the 50-200MW market in the Asia/China/India region is well served by the technology. Typically this scale of power project replaces existing high speed reciprocating power stations or provides for rapid additional local/regional capacity. The final technology choice often comes down to a choice between CCGT plants and large reciprocating plants, in either simple or combined cycle.

It is the flexibility and cleanliness of gas-engine based power that is attracting most attention in the market place. The improved turndown ratio of a plant running multiple smaller units versus one or two large gas turbines is well known. This comes from both the high levels of part load efficiency of the gas engine and a simpler mode of operation by simply switching off or on individual units as needed.

Modern high- and medium-speed reciprocating gas engines are able to start, synchronise and ramp up to 100% load within minutes. This meets the requirements for secondary response level (SRL). Even faster start and loading is possible considering specific project conditions.

The plant can offer simultaneously with different engines tertiary response level (MRL) with simple or combined cycle, secondary response level in simple cycle through start from hot standby and shutdown, as well as primary response level (PRS) in simple cycle. The relatively flat fuel consumption curve across the power range allows for high levels of turndown in primary response without significantly degrading plant efficiency.  The proportions of PRL, SRL and MRL offered for despatch can be adjusted in minutes to meet the ever changing capacity requirements and pricing structures.

Power stations that can be dispatched within minutes are important assets for balancing electric system loads and maintaining grid reliability. The generating technology affects the time required for a power plant to start up and reach full load.

The internal combustion engines can start and achieve full electrical output in less than 3 to 5 minutes for HS & MS engines to meet the stringent customer requirements – providing flexible, quick start capability. However, CCGT solutions can take over 30 minutes to achieve full electrical output.

Benefits of gas-engine based power stations
Once financing is in place and notice to proceed given, time becomes critical. In a recent Indian public tender, two developers offered reciprocating solutions with a project timeline of 14 months to commercial operation date (COD) whereas the CCGT solution required 18 months. Where regions suffer from irregular power supplies the reduced timeline can allow for faster local economic development.


The shorter lead time of the core reciprocating engine reduces project duration. However, it is the use of repeated modular elements for the plant design that enables parallel working at site to reduce build duration. The use of pre-tested modules shipped to site reduces on-site construction time and the risk of extended plant commissioning duration. Furthermore the size of the individual power units means that major items such as auxiliary modules, chimneys, cooling and charge air systems can be completed and tested in a factory environment, broken down for shipping in standard 40ft containers and delivered to site ready for installation and operation.

Gas engines exceed 48% electrical efficiency and as a result they are entirely practical for baseload applications where they replace existing simple cycle gas turbines or high-speed engines, with engine efficiencies of 35-45%.

As total efficiency becomes more critical than capacity, the addition of combined cycle steam turbines as a further development phase can lift the plant electrical efficiency to more than 50%. In CHP applications the overall plant efficiency can easily reach more than 95%. Planning for this upgrade at the start of the project allows for the space and services and has small additional cost.

Partnering gas engines with renewables
The power sector is undergoing one of the most profound transformations since it began. Developing economies are now dealing with rapid industrialisation and the associated need for increased electrical power. Domestic and industrial consumers across the region are also demanding cost effective, reliable access to power. Many countries in the Asian region are starting from a low basis of power demand and must satisfy the triple challenge of capacity growth, transmission infrastructure development and distribution penetration.

Many of those countries have low levels of electricity penetration and lack a nationally interconnected grid infrastructure. This makes it doubly difficult to grow variable renewable energy (RE) faster than the overall electrical capacity growth. In these cases, large-scale off-grid renewables can be a means to quickly bring power to consumers, although the delivery of stable capacity and the future integration into a national network must be carefully planned.

The microgrid concept will be a natural fit between the reciprocating engines and renewable source of energy, supported by battery storage. Such a pairing increases system efficiency because when the sun is shining and the solar panels are producing electricity, the fossil fuel generators do not need to run. Also, the batteries can be charged. This reduces fuel costs, emissions and generator maintenance needs. And when the sun is not shining, the customer has alternative sources of energy to ensure reliability. The reciprocating engines are flexible, can run any time on fuel and are not limited to daylight hours when the sun shines. Shorter periods can easily be bridged by batteries without starting the reciprocating engines.

The trend towards alternative and sustainable energy sources has led in recent years to the widespread use of photovoltaic solar farms. Solar energy is eco-friendly and literally inexhaustible but is not continuously and predictably available. Darkness and weather conditions can lead to fluctuations in the energy supply. One answer to this problem is provided by fossil fuel generator sets that produce energy reliably around the clock. Working together with project partners, RRPS and MTU Onsite Energy can offer its clients bespoke solutions that combine the advantages of e.g. photovoltaic solar arrays, battery storage and gas power plants to create a reliable and sustainable system.

 

Beside little space requirements, reciprocating engine generators are also valued in a microgrid for their ability to ramp up quickly. This speed-to-service not only minimises or negates power outages, but also assists in microgrid optimisation.

Optimisation refers to the ability of an advanced microgrid controller to leverage the microgrid’s various resources for best economics. Sometimes referred to as the ‘brain’ of the microgrid, the advanced microgrid controller is the software that orchestrates all of the microgrid’s resources and enables it to disconnect and re-connect to the central power grid.

The controller constantly calculates the best or optimal mix of resources for the microgrid to use based on energy prices, fuel availability, weather and other factors. Because the reciprocating engine can start and stop quickly – and typically has a ready source of fuel – it’s a flexible tool the controller can leverage. For example, the reciprocating engine generator might be quickly called into action if there is a sudden drop in solar or wind generation. It also may serve as a tool for peak shaving or other forms of demand management.

Regardless of the pace of increase, it is clear that the scale of variable renewable capacity will increase and it is likely that the share will also increase. A rule of thumb is given that there is an economic and operational ceiling on the maximum capacity share that can be provided by variable renewable energy that is roughly equal to the capacity factor. While there is much discussion around this suggested maximum economic and technical ceiling, the impact of the variable renewable capacity, at much lower levels of penetration, is already being felt on networks around the world.

A flexible future
The flexibility of gas engine technology is expected to occupy a considerably larger role in maintaining grid stability. The transition from big, centralised peak power stations and gas turbines to smaller units suggests more engine-based power plants in the future.

The expectation of increased demands for flexibility and efficiency, lower emissions and lower costs is set to increase technical demands on gas engine technology.

Flexibility in the power system to integrate more renewable wind and solar, seems to be the top priority and need of an hour. And also the demand of electricity is less stable than before. It’s getting more and more peaky. And to follow those peaks as precisely as possible, we need fast and flexible generation, and this is what the reciprocating engines are capable of.

Gas is well suited as fuel to support balancing needs. However, over the next years the Asian region will become a major importer of gas as consumption grows at a faster pace than production. To achieve the joint goals of increasing geographical diversity of economic activity, and a reduction in the carbon intensity of the economy, considerable gas infrastructure will be required to be built alongside enhanced electrical network infrastructure to dispatch variable RE to industrial centres. The decentralised power generation approach is in favour of gas-based balancing power stations to be located at new industrial zones, while balancing the effects of variable RE. In combination with, for instance, batteries for energy storage, high efficient microgrids are formed. As a further advantage, such microgrids can be established much faster to ensure reliable energy supply to these areas instead investing in long lasting and expensive transmission and distribution network projects.

For gas-based balancing, pure gas engines offer a path to quickly delivering new capacity in remote areas. The modular design and construction of gas engine power plants allows for better matching of financing and revenues while being able to deliver the needed grid support services. The start stop and loading performance as well as the high level of part load efficiency even in open cycle enable this type of plant to match the needs of the region.

Dr Harald Gretscher is with MTU Onsite Energy in Germany and Kapil Verma is with MTU Onsite Energy in India.
www.mtu-online.com

 







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