Insert 1: Technical Outline of nGen Systems
The diagram in Figure 3 outlines the generic structure of an nGen System and its key components. It is important to note that this diagram does not disclose the specific features that enable higher efficiencies than in the case of internal combustion devices. Its only purpose is to present the generic structure of an nGen System. One aim of Figure 1 is to emphasise the modular character of the system.
An nGen System typically comprises a front-end Compressor, a Burner and/or Heater, a compressed air (or other gases) expansion device (Expander), a Recuperator to recycle waste heat. The frontend Compressor and Expander are coupled together. In this fashion the Expander powers the front-end Compressor. The shaft of the Expander is also coupled to other mechanical devices and/or a rear-end power Generator to provide on-site utility services. Waste heat from the Burner and/or Heater is also recycled.
The Compressor takes ambient air and produces a small flow of compressed air that is pre-heated in the Recuperator and then heated further in the Burner and/or Heater. In doing so the combustion gases are preferably not mixed with the flow of compressed air. The heat from combustion is transferred to the compressed air through the heat exchanger components of the Burner or of the Heater. Optionally when customer requirements warrant it the fuel may be injected in the compressed air flow and the resulting combustion gases and excess air may be supplied to the Expander.
When a gas is heated at constant pressure it expands. The flow of compressed air suitably heated expands substantially as it passes through the Burner/Heater. This expanded flow is then injected in the Expander to power it.
Suitably configured the front-end Compressor requires substantially less power than what is developed by the expansion.
This leaves a significant amount of power available at the crankshaft or axle for mechanical applications (e.g. further compression for compressed air energy storage, waste water recycling through reverse osmosis, transport applications) or to power an electrical generator.
What are nGen Systems?
Recent photos taken of the nGen system development and construction.
nGen Systems are not engines, or mere cogeneration or tri-generation units. Instead they are just that – systems. They are made of individual modules that can be integrated on site in a variety of ways to match specific customer demands. These individual modular components of nGen Systems are well proven. Some are commercially available “off-the-shelf” while others can be readily manufactured based on well known existing designs and techniques.
The fundamental innovation in our technology is in how those components are specified and integrated into an nGen System in order to supply end user requirements at costs and efficiencies that legacy infrastructures simply cannot match.
nGen Systems are based on the well known recuperated Brayton Cycle (see Figure 3 below and Insert 1 right). Most engines in the world are compressed gas engines. The compressed gases are used to push mechanical devices such as pistons or turbine blades (generically called “expanders” because the gases that push them do so by expanding from high pressure to lower pressure). What makes these engines largely inefficient in the specific context of meeting on-site customers’ demands is the combustion of a fuel inside the engines as the means of primary energy input.
Figure 3 – Generic Diagram for nGen Systems
After over a century of development this approach has reached its potential efficiency limits. It is generally recognised in the co-generation domain that the best diesel engines currently achieve only 45% efficiencya. Gas turbines can achieve higher efficiencies but only at very large sizes (e.g. central power station sizes), well beyond most on-site requirements (and in central power stations most of the waste heat remains unused). At on-site scales (typically from 3kW to about 1 MW) gas turbines are significantly less efficient than the positive displacement expanders used in nGen Systems. Furthermore, in cogeneration or tri-generation installations based on reciprocating piston engines or gas turbines, the heat and electrical power outputs remain closely linked rendering difficult variations of individual energy streams.
In nGen Systems the primary energy input takes place outside the mechanical devices. This move enables substantial increases in mechanical and overall efficiencies. It also opens the way to use in modular fashion component technologies with low operation and maintenance cost and that have been in use for over 100 years. This same move to external combustion or heat supply frees up energy sources. Where internal combustion engines can use only one type of fuel (e.g. petrol, diesel or natural gas), an nGen System can be switched from fuel to fuel with only minor adaptations. This opens the way to being able to choose lower cost fuels as markets evolve.
The high efficiency capability of nGen Systems is a direct function of operating temperatures at the level of the positive displacement expander. While a wide range of operating temperatures is feasible from above 100oC upwards, at the present stage, operating slightly over 1,000oC enables using readily available materials and achieving approximately 55% electrical efficiency.
nGen Systems have few moving parts and no valves (unlike piston engines). The core components for the compressor and expander modules have been in use in other industries since the 1940s. They have required very low maintenance and are known for their longevity.
a See for example, http://en.wikipedia.org/wiki/Diesel_generators indicating that “a modern diesel plant will consume between 0.28 and 0.4 litre of fuel per kilowatt-hour at the generator terminals”. With about 38.1 MJ/litre of Diesel fuel, this translates into efficiencies of 25% to 36%. In our experience, 45% efficiency corresponds to best practice at optimal operating regimes for the best commercially available equipment.
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