Particle Heat Exchangers (PHX)

The Concept: Spread small particles through a warm gas stream to absorb heat, collect the particles, spread them through a cool feed gas to preheat it, collect the particles, and continually repeat the process.

The Advantages: The particles provide an extremely large heat transfer area, rapid and direct heat exchange between the solid and the gas, low contact thermal resistance, and direct transport of heated material from the warmer to the cooler gas stream. The technology has low capital cost (compared with a standard tube and fin heat exchanger) and low energy costs (low flow resistance compared with standard technology), and can be retrofitted to existing facilities. The use of PHX's would save significant energy $ for industry and offer an excellent ROI.

Technology Details: A particle direct contact heat exchanger is being developed to collect and reuse waste heat from industrial and power generating processes. Large numbers of evenly spaced small particles falling through a gas stream are used to efficiently transfer heat from a warm, combustion product gas stream to a cool air intake. The particles are conveyed to the top of a warm exhaust duct, where they are dispersed and fall through the moving gas, absorbing heat. The warm particles are then transferred to a cool duct, where they repeat the heat transfer process in reverse, warming the combustion intake flow. The particles are sized to fall at the proper speed, and the particle field is sized to absorb most of the heat in the warm gas flow and lose this heat again to the cool gas flow.

Technical Advantages: A particle heat exchanger (PHX) has a much higher thermal contact area, greater utilization of the gas heat exchange capacity, lower pressure drop, greater heat exchange dynamic range, and greater resistance to fouling than conventional heat exchangers. PHX's transfer heat directly to the gas; there is no intermediate metal boundary with its associated skin and temperature gradient losses. There are many types of what are termed air heaters on the market (heat wheel [1], tube heat exchangers, etc.) that perform a similar function but less efficiently. A PHX is also much simpler system than standard heat exchangers. The pressure drop of the PHX is reduced compared with standard heat exchangers because the significant pressure drop of high L/D radiator fins paths is replaced by low-drag particles dispersed in the flow. Uniform particle field heat exchangers have already been extensively investigated and proven feasible in other applications, both experimentally and theoretically. Particle heat exchanger experiments have demonstrated multi-100 kW low pressure drop heat rejection. Particle heat exchangers have limited application to enclosed, relatively well behaved flows - exactly the application of recovering waste heat. Another potential major technical advantage is the ability to tolerate (acidic) condensation. The heat transfer medium can be easily cleaned or replaced; an unusual characteristic. A major practical advantage is the ease of retrofit compared to competing devices.

Technology Questions and Answers:

1. Why are Particle Heat Exchangers superior for extracting low quality heat?
   • Higher thermal contact area.
   • High utilization of the gas heat exchange capacity.
   • Low pressure drop.
   • High heat exchange dynamic range.
   • Direct heat transfer to the gas; no intermediate metal boundary skin and temperature gradient losses.
2. Why are Particle Heat Exchangers not already in use?
   • Industry conservatism.
   • Lack of familiarity with concept and its advantages.
   • Lack of a practical demonstration.
3. Is this a Proven Technology?
   • Yes - but as droplet heat exchangers for which a good application has yet to be found.
   • The technology has over 50 years of both theoretical and experimental development behind it.
4. What experience has Thoughtventions had with Particle Heat Exchangers?
   • Modeling development of high altitude UAV droplet heat exchanger. (Including a current DARPA program)
   • Broad heat exchanger experience.
   • Extensive background research and a number of related proposals written.
5. What are the responses to standard concerns with respect to this approach?
   • Relatively large particles of uniform size are used - no particles are lost to the air flow.
   • Particle material is chosen to tolerate corrosive environments.
   • Condensation is allowed - the device can operate effectively with liquid films on the particle surfaces.
   • Particles can easily be washed periodically to prevent fouling; there is no fouling in the flow from massive plate and fin heat exchangers.

Benefit Analysis

Economic Benefits: In 1980 it was estimated that electrical generation and industrial sectors annually rejected on the order of 2 x 1010 GJ of energy to the environment, equivalent to about 22% of the total energy consumption in the US. Typical industrial heat rejection is in the range of 10 - 100 MW, and about 1/4 of this is in waste gas heat. Based on heat exchanger effectiveness of droplet heat exchangers, increases in heat transfer efficiency on the order of 10% or higher are expected compared with current technology such as a rotary regenerator (heat wheel). The potential energy and $ savings are huge.

Social Benefits: Improved waste heat reuse would decrease energy cost and reduce thermal pollution. Power demand and power costs would be reduced, reducing the financial burden on consumers of both power and products.

• Specific Groups That Would Benefit: The materials processing and power industries would be the prime direct beneficiaries of this technology.
• The Product: A Particle Heat Exchanger retrofit package will be provided to transfer heat from a warm air exhaust duct to provide preheating to a cool air intake.
• The Market. The primary markets will be the power industry and the materials processing market. At a cost of 50-100k$ per unit (for smaller units), with many thousands of potential applications, the total market potential will exceed 100M$. The wild card is whether these systems can be applied to heating and cooling of major commercial buildings - that would result in a much larger market.
• Likelihood of Marketable Product. Based on the return on investment and time for payback of the device cost this product should be highly marketable. The problem will be that these are basically conservative industries that will require practical convincing to install such a system. Based on the performance of a demo unit installation we expect to overcome initial conservatism. The first demonstration packages must demonstrate significant energy savings and ease of operation and maintenance, as well as standard positive return on investment. The initial customer target base will be mid-sized companies that are motivated and receptive to cost-saving modification of their physical plant.

Commercialization Status

Commercialization Hurdles. The problems to be overcome for developing and implementing a technically novel waste heat recovery system include 1) the technical demonstration of the concept, 2) demonstration of the economic gains from implementing the technical solution, 3) convincing a significant user to install and operate a pilot facility, 4) demonstrating large benefits from pilot plant operation, and 5) establishing broad application in the process industry - market penetration. The major technical problem that PHX is that conventional heat exchangers performing waste heat recovery have difficulty exchanging heat at the low temperature differences required, they introduce significant pressure drops in the gas streams where they are used, and they are expensive and massive. Competition. There is currently nothing on the market today that targets the low quality waste heat that PHX's can reap. For higher temperature heat transfer/waste energy recovery, the market place is full and mature.

Contact Stephen Bates (thought@Thoughtventions.com) with questions.

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Last updated: July 2015