The humble stationary genset is never going to be the sexist piece of kit, often criticised as highly-polluting and inefficient. However, a novel waste heat recovery technology promises to turn that perception on its head by increasing power, improving fuel efficiency and reducing emissions. Paul Dowman-Tucker explains.

The diesel (reciprocating) generator set market remains fast growing and vibrant, representing the fastest selling and least expensive of the distributed generation technologies available. Applications including prime, baseload power generation (powergen), backup/standby power and temporary installations all continue to see increased sales, and this is anticipated to continue well into the future, particularly in the developing world. A recent report by Navigant Research states that there remains steady growth in sales of reciprocating engines, burning a range of fuels for powergen including diesel, natural gas and biogas. The latter two fuels are particularly pertinent in the context of rising exploration for shale gas, and increasing development and exploitation of renewable gas sources (such as anaerobic digestion and landfill gas.

Sectioned view of the turbogenerator
Sectioned view of the turbogenerator

This continued growth demonstrates that the economics for reciprocating gensets work, but in the face of perennially rising fuel costs engineers persist in exploring means to recover a significant proportion, sometimes in excess of 60 per cent, of the calorific value of the fuel being lost to the environment as waste heat.

Many technologies have been explored and developed to recover some of the waste energy, predominantly from the exhaust, but none of them have yet seen widespread uptake. Examples include mechanical turbo-compounding, steam cycle solutions, organic Rankine cycle (ORC) solutions, thermoelectric materials and electric turbo-compounding. The key challenges faced include, cost and the resulting lengthy payback period, reliability and general development maturity in the context of a conservative industry. Additionally, difficulties associated with space and weight requirements also constrain the range of potential applications, although perhaps less so in stationary powergen.

Novel waste heat recovery tech

Bowman Power Group, based in Southampton, UK, has been working for the last nine years on technology to recover waste heat energy by means of electric turbo compounding. Bowman ETC systems have seen considerable success to date in stationary powergen applications working on two different value models – power boost (of up to 13 per cent), or fuel saving (of up to 8.5 per cent).

The technical principles by which the system operates are simple on the face of it. A power turbine stage (gas expander) is fitted into the engine exhaust stream, either in the waste gate flow if available and appropriate (parallel application, or downstream from the engine turbocharger turbine stage) series application. This power turbine is close coupled to a compact, high-speed, high-efficiency alternator which produces high-frequency AC electricity which is converted into grid quality 3-phase AC and added to the host genset output for connection to the load, be it a direct load, or local or national grid. This combination of a turbogenerator and the matched power electronics make up the Bowman ETC system.

Comparison of fuel consumption
Comparison of fuel consumption

In parallel applications, the presence of the turbogenerator does not have an impact on the engine exhaust aero-thermal characteristics, and it is therefore simple to integrate, requiring no turbocharger modifications. In this application model, it is possible to utilise a proportion, or all of the wastegate flow, depending on requirement, payback economics etc, utilising bypass features and the control system embedded within the power electronics.

In the more common series application, the standard engine turbocharger is often replaced or modified to firstly improve its efficiency (this gives some economy benefits in itself) and secondly to ensure that, in conjunction with the turbogenerator, the overall aero-thermal characteristics of the engine exhaust system continue to operate within acceptable limits. It should be noted that the overall system back pressure is normally increased by the application of the ETC system, but in the applications developed to date this has not proven to cause a problem with engine durability, and in fact a recent study into the ETC system by Ricardo, has shown that there can even be benefits from the additional back pressure in cases where the host genset engine is fitted with an exhaust gas recirculation (EGR) system to help manage emissions. Ricardo’s overall conclusions were: The Bowman Power turbogenerator system clearly demonstrated the potential for significant fuel consumption improvements across all the engine categories assessed (0.5-2 MW), Furthermore, as the turbogenerator system is focused on continuous rating applications, the risk of increased exhaust temperature and pressure should be minimal.

Rigorous testing applied

The turbogenerator, power electronics and sometimes also the turbochargers are designed, manufactured and tested at Bowman’s Southampton premises. The design of the TG has been developed over time by the experienced and expert engineering team within the company, and for some aspects, in conjunction with independent consultants. The turbine itself is highly efficient and uses fixed nozzle guide vanes, which can be changed to optimise the match of the TG to each application.

Schematic diagram of the Bowman ETC system
Schematic diagram of the Bowman ETC system

The power electronics are also developed in-house and delivers an overall electrical conversion efficiency of approximately 98 per cent, ensuring the maximum possible benefit to the end user. The power electronics are modular in concept, centred on a standard converter module that can be configured for the different functions in the power electronics and grouped together in various numbers to provide the overall capacity necessary for the application in hand. In the face of developing grid connection code requirements, Bowman is currently going through the process of certifying the power electronics against the challenging German market standards including those of VDE and BDEW.

Achieving significant fuel savings

The Bowman ETC technology has been tested in a variety of applications, both on vehicles and for stationary powergen, and the benefits are tangible. For example, testing conducted of on a Bowman ETC system fitted to a 900 kW (continuous) containerised genset demonstrated a peak fuel saving of 8.5 per cent for the same electrical output compared with the baseline genset.

The fuel consumption data from these tests highlights that, in practice, the fuel saving is consistent across a wide range of power outputs. This latter point is particularly important when considering applications which have variable outputs, and also in comparing with other technologies which have a narrow band of optimal performance.

Noting the barriers to take up of waste energy recovery technologies, it is important to state that the cost of this technology has been driven hard through value engineering activities and the take up in certain markets at reasonable volumes. The Bowman ETC system is delivered for less than £1000/kW ($1558/kW), which compares very favourably with competing technologies, such as ORC, which can cost in the region of £2000-5000/kW. This enables Bowman to offer a system which, depending on fuel cost, can achieve a payback of significantly less than two years.

Key value models

It is worth considering a couple of examples in order to explain better the two key value models of this technology. Taking the power boost model first, we can examine the application of the Bowman ETC system to a Scania DC12 based genset packaged for use in the agri-tech market where feed-in tarriffs (FITs) are available for installations burning renewable fuels to generate power for the grid.

Figure 4: An example payback diagram for Bowman ETC system
Figure 4: An example payback diagram for Bowman ETC system

SCHNELL Motoren AG is a pioneer in biogas plant construction, based in Amtzell in southern Germany near the Swiss and Austrian borders. Originally an end-to-end provider, it has evolved into the market and technology leader for packaged dual fuel CHP units.

SCHNELL wanted to offer end-users the benefits of a more efficient genset. In Germany, the FITs increase for all the power generated when operating above 45 per cent electrical efficiency. The SCHNELL 265 kW CHP unit is based on the Scania DC12 engine, an 11.7 litre displacement, six in-line cylinders and 1500 rpm machine. In 2009, SCHNELL integrated a 30 kW Bowman ETC system within the CHP unit. The result was that the SCHNELL 265 kW CHP unit was the first dual fuel CHP unit to operate with an electrical efficiency higher than 45 per cent, achieving 48.3 per cent in reference conditions testing.

Kalgoorlie Power Systems (KPS), a subsidiary of Pacific Energy, is a leading provider of power generation infrastructure to the mining and resources sector in Australia. The business executes a build, own, operate and maintain commercial model, with in excess of 250 MW of contracted capacity at 23 mine sites across Australia. KPS tenders to provide solutions to its resource sector client base. Critical to competitive tender success and client retention is the provision of the lowest fuel consumption generation infrastructure available.

In 2007, KPS and Bowman agreed to jointly develop an optimised Bowman ETC system solution for retrofit to the Cummins KTA50 G3 engines used in the KPS power generation fleet.

Upon successful completion of the development and trialling programme, which demonstrated a consistent 7 per cent fuel consumption reduction, KPS and Bowman entered into a long-term, exclusive agreement in Australia, to supply TGs. To date, KPS has purchased 70 Bowman ETC systems and is advancing the roll-out of the technology across its power generation fleet.

The above information and examples describe applications delivered to date, however the technology and its application continue to be developed by the engineering and operations teams at Bowman. In working with the current and future client base, a number of opportunities to improve the integration of the ETC system with the host genset have been identified and solutions to these challenges are now being trialled on prototype installations.

The first and most important of these is the ability of the system to safely shut itself down, and take itself out of the loop, in case of a failure of some kind which will not impact the operation of the host genset. Availability – ‘up time’ – is absolutely critical in the vast majority of stationary powergen installations and although the ETC system is key to overall efficiency of the plant, it is peripheral to the primary function of generating and exporting power.

Recognising this, Bowman has worked on integration of bypass and regulating valves, with sophisticated control algorithms embedded in the PE to enable the system to be essentially non-invasive in functional terms.

Other areas of development relate, in the main, to packaging and integration modularity to suit different makes and models of host engine. Many diesel gensets, for example, are packaged within ISO containers, and therefore either a ‘bolt-on’ containerised ETC system is required, or it is necessary to integrate all the components directly on or alongside the engine within the container. In the case of on-engine integration, trials are underway to rationalise the oil and cooling systems of the genset and ETC system to optimise the packaging from a space and weight perspective, and also improve the cost efficiency of the system further.

Other applications

Development is also underway in respect to broadening the range of application of the ETC technology beyond stationary powergen and into other environments and opportunities. There are other industrial processes which feature high-temperature and pressure waste gas streams which could be harvested for electrical energy, as well as gas flow pressure reducing requirements which also offer potential energy recovery opportunity. Furthermore, powergen in marine and offshore applications is frequently operated on a base load basis, offering short payback opportunity when coupled with the often high fuel logistics costs.

It is clear that the reciprocating engine based genset has a long future ahead, burning a range of fuels in order to generate power against an ever increasing demand profile. While highly mature, the engine technology remains characterised by relatively low overall thermal efficiency, and therefore it is imperative to continue to seek means to improve this by recovery of useful energy from the waste heat and pressure.

Paul Dowman-Tucker is director of Engineering at Bowman Power Group. For more information, visit www.bowmanpower.com.

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