BMW provides an update on waste heat recovery projects; Turbosteamer and the Thermoelectric Generator

30 August 2011
 Research project Turbosteamer, Generation 2. Click to enlarge.

Despite improvements in engine efficiency—e.g., with technologies such as direct fuel injection, variable valve timing, exhaust-driven turbochargers, brake energy regeneration and Auto Start Stop function—about 60% of the generated energy is still lost, half of it being exhaust heat, with the remaining half as heat absorbed by the engine cooling system. Finding ways of recovering this lost heat energy has been one of the major goals being pursued by engineers working on BMW EfficientDynamics for the future.

The BMW Group is involved in several projects, each with different approaches to recovering dissipated heat energy, and at various levels: in research, pre-production and series development. The company says that some of the most promising projects are the Turbosteamer (earlier post); the Thermoelectric generator (TEG) (earlier post); engine encapsulation; and a waste heat exchanger for oil heating.

The Turbosteamer and Thermoelectric Generator (TEG) projects are focused on generating electric current from waste heat to improve overall engine efficiency, but each project follows a different approach and time frame. There is great potential for considerable fuel savings if the electrical energy required by all of the systems in an automobile can be produced using waste heat rather than relying solely on the vehicle’s generator, the company notes.

 Research project Turbosteamer: comparison of the heat exchanger generation 1 (top) and generation 2 (bottom). Click to enlarge.

Turbosteamer. In the Turbosteamer Project, BMW is working on a heat recovery system that is based on the principle of a waste heat recovery process already practised on a large scale in modern power generation plants. Large gas and steam power stations combine the principles of a gas turbine and a steam circuit to achieve a significantly higher level of efficiency. The gas turbine process is the first phase of the energy conversion and serves as the source of heat for the downstream steam cycle in the second phase.

The BMW turbosteamer is based on this two-stage stationary power generation method, but reduced in scale and design to form a component that can be used in modern automobile engines.

Researchers proved the feasibility of this technology in December 2005 with the unveiling of the first-generation turbosteamer, which was based on a maximalist approach: they designed a dual-cycle system. The primary element was a high-temperature circuit that employed a heat exchanger to recover energy from the engine exhaust gases. This was connected with a secondary circuit that collected heat from the engine cooling system and combined this heat with the high-temperature heat from the primary circuit to create lower temperature heat.

When this design was laboratory tested on the four-cylinder gasoline engines produced by BMW at the time, the dual system boosted the performance of these engines by 15%.

In order to further develop the system for use in series production, BMW focused on reducing the size of the components and making the system simpler to improve its dynamics and achieve an optimized cost-benefit ratio. Thus researchers focused on designing a component having only one high-temperature circuit.

A heat exchanger recovers heat from the engine exhaust, and this energy is used to heat a fluid which is under high pressure. This heated fluid then turns into steam, which powers an expansion turbine that generates electrical energy from the recovered heat.

—Jürgen Ringler, Team Leader for Thermal Energy Converters at BMW Group Research and Technology

For the latest generation of the turbosteamer, engineers developed an innovative expansion turbine based on the principle of the impulse turbine, which offered many advantages in terms of cost, weight and size when compared to earlier concepts.

We have made great progress toward achieving our original goal, which was to develop a system ready for series production within about ten years. When completed, this system will weigh only 10 kg to 15 kg and will be capable of supplying all of the electrical energy required by an automobile while cruising along the motorway or on country roads.

—Jürgen Ringler

Under these conditions the developers are sure that the average driver will be able to reduce fuel consumption by up to 10% on long-distance journeys.

All of the system components developed on the test bench have been configured to form a module that can be integrated in vehicles. This has been done successfully by installing a mock-up system in the BMW 5 sedan.

Thermoelectric generator. BMW says it also has made considerable progress in the Thermoelectric Generator (TEG) Project that is also focused on series production. The two alternative systems developed to date differ in their positioning in the vehicle—one unit is designed for the exhaust system, while the other is intended for the exhaust gas recirculation system. The development phase focused on integrating units in the exhaust system has led to considerable component improvements, especially in terms of weight and size.

 TEG integrated in the exhaust manifold. Click to enlarge. TEG integrated into the exhaust gas recirculation system. Click to enlarge.

The thermoelectric generator converts heat directly into electricity. The engineers of the BMW Group basically refined a technology that has been used to power space probes for more than four decades by NASA, the aeronautics and space agency of the United States. The principle behind this technology is known as the Seebeck Effect, namely that an electrical voltage can be generated between two thermoelectric semiconductors if they have different temperatures.

Since the percentage degree of efficiency of TEGs was rather low, this technology was considered unsuited for automotive applications. However, in recent years progress in the area of material research has led to discoveries that have improved the performance of TEG modules.

The first step taken by engineers was to integrate a thermoelectric generator in the exhaust system to generate electrical current. The first such system was shown to the public in 2008 and delivered a maximum of 200 watts, which was relatively low in terms of power efficiency. But the use of new materials and improvements in the weight and size of the TEGs led to rapid new developments, so that the latest generation of TEGs installed in the exhaust are capable of generating 600 watts of electrical power, and it will not be long before the goal of 1,000 watts is reached as research progresses. The current prototype—a BMW X6—was built as part of a development project funded by the US Department of Energy.

Then in 2009, the BMW Group unveiled an alternative development in this project. Rather than installing the TEG as a separate module in the exhaust system underneath the vehicle, engineers decided to integrate the TEG in the radiator of the exhaust gas recirculation system. In this configuration, customer testing has shown that 250 watts can be generated while CO2 emissions and fuel consumption are reduced by 2% at the same time.

This energy recovery system also offers some interesting added benefits, such as supplying the engine or passenger compartment heating with additional warmth during cold starts. BMW says that the thermoelectric generator is an ideal counterpart for BMW EfficientDynamics Brake Energy Regeneration. While the brakes generate energy during deceleration and stopping, the TEG functions at its best when driving is really exciting—during acceleration. Researchers forecast that TEGs will lead to fuel consumption savings of up to 5% under real everyday driving conditions in the future.

Heat management and BMW EfficientDynamics. While some features of BMW EfficientDynamics, such as brake energy regeneration or the Auto Start Stop function help reduce consumption when decelerating or during idling periods, intelligent heat management can do the same when the vehicle is being accelerated and driven. In the future, even before starting the car, insulation and encapsulation of the engine compartment will ensure that the temperature of the drive train is stabilized by residual heat, thus shortening the cold start phase. An exhaust heat exchanger will also keep gearbox oil warm to reduce friction and fuel consumption as well.

Depending on the vehicle environment and driving habits, heat management can deliver measurable benefits for specific driving scenarios. For both short and long-distance driving various features can reduce fuel consumption. Insulation of the engine compartment, gearbox oil heating with exhaust heat exchangers installed with gasoline engines, or the heating function of the exhaust heat exchanger for diesel engines are features that are well-suited for vehicles that are predominately driven over short distances, BMW says. During longer journeys the thermoelectric generator or turbosteamer add to that. And by utilizing synergy effects, heat management can play a role in reducing CO2 emissions in the future.

If 60% of the fuel energy goes into heat, recovering 50% of it could about double the overall vehicle efficiency. Large gas guzzlers could do 30 mpg instead of 15 mpg. Imagine all the energy wasted in our vehicles i.e. 60% of 9+ M barrels every day or about 5.5 M barrels/day. At $100/barrel, that a lot of $\$/year.

Recovering 50% of the wasted energy could pay off most of the current national debt in a few years.

Yeah, this is a good idea...if you want to keep ICEVs around for another generation. There are always going to be some roles for ICEVs but the sooner & more we can switch to BEV/HEVs the better.

Heat is a very unrefined form of energy.
The trick is to convert that into some more refined form, like electricity or motive force without too much cost, complexity and size.

But there is no harm in trying.
This is where a PHEv can work well - you can use the waste heat from the ICE to heat the cabin and oil etc - if you are electric only, you have to use battery power to heat the car, which is a waste of expensive batteries.

There is a better mechanism to recover waste exhaust, they can do methanol with it right from the exhaust with a methanol maker and after we recirculate the methanol right at the input for an infini mpg car or truck.

@HarveyD
As mahonj already indicated, heat is a low form (the lowest!) of energy. The laws of nature tell you how much you can recover. In this case, the second law of thermodynamics is of importance. The efficiency in the Carnot cycle is the ultimate limit. The lower the temperature, the less you can recover. The temperature of the exhaust is the highest of the waste energy that we can work with. Still, it is much lower than the ~2 000 K we have in combustion. Recovering 50% of that energy is not possible even in an idealized cycle according to the laws of nature. Practical aspects give further limitations. A level of roughly 10% recovery of waste heat (yields ~15% power increase) is more realistic but even at that level, it is interesting. BMW mention fuel consumption reductions of 10% for the Turbosteamer and project 5% (2% demonstrated) for TEG. If the cost relation would also be a factor of 2, the choice between these options would become interesting.

I suppose that you refer to dissociation and/or reformation of methanol. Yes, this is another option for waste heat recovery. However, it is best suited for fuels as methanol and dimethyl ether. Ethanol might also work (at lower efficiency) but the complexity and lower potential (even lower efficiency) with gasoline and diesel fuels makes is uninteresting for these fuels. Thus, we would have to introduce new fuels on the market to take advantage of this option.

Like most wastes, it is far easier to reduce or eliminate heat wastes at the source. There are a few good examples already.

1. LED, OLED and WOLED lights vs incandescent light bulbs.

2. BEVs vs ICE vehicles.

3. Ultra light weight vehicles vs heavy 3-ton monsters.

4. e-trains vs large diesel trucks

5. New SEER 26+ Heat pumps vs many existing inefficient AC and furnaces.

6. Improved houses and commercial buildings to reduce heat/cold lost.

By reducing heat waste we normally reduce oil imports and GHG at the same time.

@HarveyD
Yeah, but nothing on your list has any direct connection with heat losses from the engine. You have listed complementary measures. Try to stick to the topic!

Reducing heat loss at the source (i.e. from the engine) is best accomplished by increasing engine efficiency. A lot of engineers already work on that task.

With great success.. most 1960 small cars had about the same mpg than today's. The few real winners for last 10 years or so are the Prius, Volt, Leaf, Miele, Tesla .... they have more or less replaced the inefficient ICE (60% heat waste) with e-motors with a lot less heat loss.

P.XX...heat loses is a result of bad engineering in many places, including ICE used in most vehicles. ICE is an inherently low efficiency technology with huge heat loses. It will never get much better than 55% efficient while e-motors can get 95+%. Secondly, by placing e-motors directly in ultra light weight traction wheels (why not all four), mechanical transmission loses are eliminated and, with quick charge batteries, you can recover a major portion of braking energy instead of wasting it with costly mechanical friction brakes currently used on all ICE powered vehicles.

All the R&D currently being used to extend to life of a dying technology (ICE) could be better used to improve and lower the cost of on-board e-storage units as soon as possible and promote the switch to BEVs.

The new Volvo quick charge electric city bus is a leading example of what should be done.

@HarveyD,
"The report of my death has been greatly exaggerated!" an ICE would be saying to you. You cannot directly compare the efficiency of an E-motor to an ICE.

Until low-cost fuel cells that can be made to run on any kind of combustible fuels, we will need the affordable ICE, best be used in combination with the E-motor in an HEV. My prediction is that the ICE will be with us long into the future. Even when battery will be cheap, the ICE will be used as a range extender in a PHEV.

@HarveyD
Bad engineering (?) First, if you are a much better engineer than those currently working on ICEs, why do you not get employed at one of the car/engine makers? Second, you do not mind where the heat losses are generated, i.e. at the electric power plant? With your current electricity mix in the USA, electric car are no better than hybrid ICEs when it comes to well-to-wheel efficiency. Start by cleaning up your electricity production before you even plan for an introduction of electric cars! In contrast, HEVs you can promote today and tomorrow, most likely also HHVs and KHVs.

The only real use for an ICE is as a range-extender using the new developments in rotary engines: SPARCS and HCCI ignition.

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