New engine could be an alternative to batteries

Engineerblogger
Dec 7, 2011



Cryogenic engines operate through controlled vaporisation of cryogenic liquids (eg: liquid nitrogen, air) through application of heat (which can come from the environment or waste heat sources). If this vaporisation takes place within a confined space then significant pressures can be generated. This pressure can be converted into work through use of any type of expansion engine. This concept has been used to power cryogenic pumps and companies like Highview Power Storage are commercialising the process at large scale for energy storage.

Whereas most cryogenic engines use heat exchangers for indirect heat transfer, the Dearman Engine is a radical refinement of this process whereby a heat transfer fluid (water or ethylene glycol) is used to transfer heat to the cryogenic liquid inside the expansion engine through direct contact heat exchange between the cryogenic liquid and the heat transfer fluid.

Liquefied Air versus Compressed Air
Liquefied air is easier to store and manage than compressed air which is already used for energy storage, and indeed powering cars. There is no need to store it at high pressure in an expensive and bulky pressure vessel: just keep it cold in a standard, insulated, low pressure tank. Liquid air/liquid nitrogen also have significantly better energy density than compressed air (four to six times) so requires far less space. In fact one litre of liquid air when regasified becomes 710 litres of air.

Dearman technology versus other cryogen power generation technologies
The Dearman Engine combines the benefits of other cryogenic power generation technologies (low cost, high energy density, zero emission and free cooling) with its key inventive step; the heating of the cryogenic liquid (the “fuel”) through direct contact with a heat exchange fluid inside the engine. This reduces the requirement and cost for (i) multi-stage expanders or (ii) large and expensive high pressure heat exchangers to turn the “fuel” to gas before it gets into the engine, the heat exchange fluid is also the medium by which we can easily add heat or deliver cold.

Additionally direct contact heat exchange can cause very rapid boiling leading to vary high rates of pressurisation; while, because the thermal source is applied and present during the expansion phase, there is the prospect of a quasi-isothermal expansion and increased work output.

Finally, along with a being a cheaper solution, as a reciprocator engine the Dearman Engine can scale down to a few kWs as well as still potentially scaling up for 10s of MW applications.

Dearman technology versus batteries and other zero emission powertrains / batteries
An assessment of a cryogenic powertrain concept using the Dearman technology and its suitability for use in a variety of automotive applications as a Zero Emission Vehicle (ZEV) has been carried out by our academic partners and industry powertrain specialists. The key conclusions were:

• The technology may be competitive for certain classes of vehicle should regulations or incentives favour the use of ZEVs. These include taxis, urban cycle buses and light commercial vehicles for urban cycles (such as delivery vans).
• Whilst performance and range are compromised compared with internal combustion engines (as are all alternative vehicle solutions such as batteries), cryogenic powertrain technology offers the prospect of two key advantages compared with current economically viable battery electric vehicles. First there is improved energy density and therefore range. Secondly there is rapid re-fuelling capability (3-4 minutes) enabling improved vehicle utilization.
• The peak operating temperature of the engine is approximately ambient so high temperature tolerance is not required; this yields potential weight and cost savings.
• Technologies are mature for handling and transporting cryogenic liquids.
• The concept of Combined Cold and Power could offer significant commercial transport advantages – e.g. chilled delivery vehicles, where the energy and capital costs and the weight of the refrigeration unit are already captured in the powertrain.
• The engine could harness waste energy from braking.

The key engineering challenge was optimizing the injection of liquid nitrogen into the cylinder but that all the technologies involved, including the LN2 storage tanks, were mature enough not to be showstoppers.

Source: Dearman Engine Company (DEC)