Going the extra mile

Written by: Tim Fryer | Published:

Recovering energy from waste heat is nothing new, but being able to do it cost-effectively on a small scale is. Starting with trains and trucks, such technology could be coming down the line.

American locomotives are fairly meaty beasts. Instead of having individually powered carriages, typically of around 300hp, as we do in the UK, a US locomotive pulls the entire train and will be an order of magnitude more powerful at around 4000hp. Thermo Dynamics Rail set up in the US to develop technology to recycle the waste heat, of which there is a considerable amount in such a powerful machine. The waste heat is fired out in the form of hot exhaust gases from the diesel engines, which is quite a useable form for the technology being developed by Entropea Labs, Thermo Dynamics’ Rail’s British subsidiary.

Entropea was set up in the UK because of the access to engineering and academic expertise - expertise that the parent company was struggling to obtain in the US. Now Entropea does the majority of the development work and the Train Dynamics Rail becomes the base for manufacturing and testing. The most advanced application is currently for a locomotive and this is likely to be installed for testing within a year.

The basic technology is the Rankine Cycle, used extensively in power stations where it is the generation process, and also steel and cement works and other applications where huge amounts of hot exhaust gases are produced.

In short, taking power generation as the example, the burning fuel heats water to a superheated steam state. This steam drives a turbine which is attached to a generator and hence electricity is produced. It is a process that can be applied to any thermal process that has waste heat, usually as exhaust gases, as a by-product, for example the steel works. In this case it is the exhaust gas that transfers its heats like the working fluid.

Rankine cycle based systems work well at the hundreds of megawatts scale, but as power levels diminish so does the efficiency. Turbines and generators need to spin very quickly to get any useful electricity out of steam and such components come at such a cost that below a certain level the financials don’t add up.

There is a technical way round this and that is to use organic fluids rather than water as the working fluid. These organics can superheat at temperatures as low as 40°C rather than the 400-500°C it takes to superheat steam (depending on the pressure). Such Organic Rankine Cycle systems have been used in static applications, such as biomass generation plants where size and temperature are both greatly reduced, mainly aimed at generating around 0.5 to 5MW.

Such organic fluids are more viscous than water and can generate higher torque and consequently a useful amount of power by turning the turbines at much lower speeds.

Entorpea’s goal is to take this one step further and bring the technology to applications that are much lower power. And mobile.

While the first likely target is the American locomotive, the idea is for construction equipment, trucks and eventually cars to have the ORC units fitted. “The problem is that the smaller the system, the harder it is to make it,” said Dr Benjamin Franchetti, managing director of Entropea Labs. “That it's going to work is, I would say, certain, because the Rankine cycle technology is not reinventing the wheel. It's just whether we can make it cost-effective - that's really the bottleneck. And I think the key to do that is to not rely on external suppliers.”

While Dr Franchetti concedes this will make his life harder, it will ultimately make the product cheaper to the point where it is viable. “What you typically see when people work in this field is that they might just do the turbine, and then they will rely on another company who do the heat exchanger, and then they will buy the generator from another company, and then they put everything together. Each one needs to make their overhead on it, so what you end up having is to have a more expensive system. Each one of us at Entropea is specialised in a specific field, and we are trying to develop our own technology in all of these sectors.”

Turbine design is one of the most challenging fields as the power range is at the lowest where turbines can be efficient

So where do the challenges lie? One comes in the heat exchanger. In large static applications this is not a problem – there is plenty of space so greater heat transfer can be achieved by just using more tubes, to put it simplistically. However, in transportation applications this has to be put in an enclosed space and in some occasions very close to other hot components. The design of the heat exchanger starts to become far more important.

“In heat exchanger design, one is really looking at the theoretical part, turbulent motion to maximise the heat transfer,” said Dr Franchetti. “It's all about creating the structured motions which hit the walls in a way that you maximise heat transfer. Higher velocities are better. It's counterintuitive, but in fact, the exhaust gases heating the walls more quickly carries more momentum, more kinetic energy, and this kinetic energy will transfer more heat to the organic fluid. If it goes very slowly, even though it spends more time going through, it carries less energy to the point that if it becomes laminar - the motion is so slow that you don't transfer anything.”

The other consideration with heat exchangers is to maximise surface area. Adding fins can do this, but they can also increase the possibility of fouling. This can not only make the heat exchanger less efficient, it can also cause sufficient blockages for it to effect the performance of the engine.

“Each of the components is equally important, but possibly the hardest one is the turbine,” claimed Dr Franchetti. “Turbines become very hard to develop in a low power range, they become very inefficient. You start to be in that grey area where do you use a turbine or use an expander, like a piston, which make sense up to hundreds of kilowatts.

“We are in that grey area where turbines start to become quite inefficient, but pistons are also not that good. So what do you use? When we are at 30kW, we are betting on turbines, but there aren't really any commercial turbines for that power generation. When we are at 300kW, I’m definitely going to say turbines, but still it's very hard. So our driving research is on the turbine development part in this field.”

Generators are also important according to Dr Franchetti: “Electric machines have been in existence for a long time but not at those speeds and those power ratings.”

And the other main component is the condenser. Another piece of the jigsaw that in static conditions, working close to vacuum conditions, is relatively standard.

“The vacuum allows for a huge energy jump, in essence, which is a huge power jump,” explained Dr Franchetti. “The bigger the difference between the pressure of the steam entering the turbine or the organic fluid entering the turbine and the pressure of the condenser, the outlet of the turbine, the more power you generate.”

In the hostile environment of transportation keeping a vacuum is not feasible, the tiniest leak would result in the integrity of the condenser being compromised and that power differential being significantly reduced. Entropea therefore take its starting point as atmospheric pressure and, like the other components, have readapted it to be applicable for the truck power ratings.

The systems are being put to the test at Brunell University where an engine rig has been set up to provide the exhaust gases needed to put the system through its paces.

Dr Franchetti concluded: “We are in the process of finding how much electricity comes out of it and how well it's working in a laboratory environment. Then on the road, this will change.”

The belief is that the system will be able to reduce fuel consumption by around 10 – 15%.

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