Joining a modified Stirling engine and an HVACR system where the HVACR refrigerant gas is the Stirling engine working gas.



Our thermal compressor is the joining of a modified Stirling engine and an HVACR system where the Stirling working gas is the refrigerant gas of the HVACR system.

For the mobile AC/R sector, the compressor runs on the wasted energy of the exhaust tailpipe of buses, trucks, vans and cars.
For the fixed HVACR sector, it uses thermal CSP, and due to the large sizes of the modified Stirling engine/compressor possible, electricity co-generation will be done.

In our modified Stirling engine/compressor, as the pistons move, they pressurise the refrigerant gas. In our modified Stirling all cylinders are hot, there are two one-way valves per cylinder opening in opposite directions (in and out valves).

In our modified Stirling, the low and high-pressure lines of the HVACR system are connected to the modified head of the Stirling. The cool refrigerant gas leaving the refrigeration unit, enter the Stirling cylinder, heats up, expands, produces work, pushing the first piston down. In the second cylinder, the same takes place. As the second piston goes down, it pushes the first piston up. The pressure inside increases to a level equal or higher than the high-pressure line of the HVACR system. At this point the second one-way valves opens and the refrigerant gas leaves the engine,/compressor pressurized. The cycle repeats itself.

So the gas laws says that P.V=n.R.T since the gas and volumes in the modified Stirling are the same, the equation can be written as P1/T1=P2/T2 where in a common AC/R system, T1 is the temperature of the refrigerant gas leaving the refrigeration unit at around 15C, P1 is the low-pressure line around 60psi, P2 is the high-pressure line around 200psi, thus the temperature T2 is 50C. This shows how little temperature/energy will be needed (sure depending on the HVACR system size, but for mobile, they are not so big. For fixed HVACR, there are plenty of roof spaces for CSP to collect sun's energy).

What are the key outcomes and impact of your solution?

For fixed HVACR systems, the best metric is to take the amount of electricity in a given country that is used by HVACR system. In most developed countries this figures around 20% of the total country's electricity. In the developing nations, this values is lower but rising sharply. HVACR system demand a high amount of energy.

For mobile AC/R, the metrics are based on the reduction of fuel consumption in vehicles using AC/R.

On average, vehicles running AC/R consumes 20% more fuel (see references). If a vehicle travel on average 10.000Km (in six months) and consumes 1 litre per 10Km, it will consume 1.000 litres of fuel. If this vehicle is using AC, 200 litres more of fuel will be consumed. There are 1.2 billion vehicles on the streets today and 94 million new one per year are manufactured.
This article published by the SAE (http://papers.sae.org/2002-01-1957/) put numbers on the fuel consumption in vehicles using AC in the US per year. "The study showed that the US uses 7.1 billion gallons (27 billion liters) of gasoline every year for air conditioning vehicles, equivalent to 6% of domestic petroleum consumption, or 10% of US imported crude oil." A simple multiplication of gallons per its costs (US$2.65/gallon) shows what the cost/financial impact (US$18,815 billion/year) Solair solution will have for the US market alone.

In São Paulo city, largest Brazilian city, there are 15.000 public city buses, most without AC, traveling some 1.0 billion Km/year and consuming 439 million liters of Diesel/year. If this fleet was fitted with traditional compressor, the increase in Diesel would be around 88 million liters/year at a cost of US$83 million. This one year cost in extra Diesel alone would pay to fit all 15.000 buses with AC using Solair solution (Extra fuel US$83 million/15.000 buses = US$5,500.00/bus, see reference).

Los Angeles Metro, has 2,221 buses running compressed natural gas (CNG) engines. CNG engines cost more to run, but saves on public health (less particulate and pollution). Metro runs the second largest public transit bus operation in the United States with nearly 400 million annual passenger boarding, and its buses log just under 1.5 billion miles a year. Solair solution can reduce this extra engine cost by reducing the cost of AC in the fleet (see references).

In the developed markets, more than 95% of new vehicles have AC/R and in developing markets, this trend is increasing. So mobile AC/R has a huge impact on GHG, pollution and costs in general.

By developing a thermal compressor, this fuel consumption increase is eliminated. This will produce immediate and large impacts on reducing GHG, pollution and costs.

What actions do you propose to realize your stated goals?

We will start with cities buses fleet because they are a major pollution source in cities, there are a huge fleet in all large cities of the world, public comfort transport is an human necessity, cities buses have large diesel engines wasting large amount of energy, lots of room to fit the complete Solair solution (thermal compressor plus the refrigerant gas storage solution for when the diesel engine is work working, but it is smart to let the bus cool (final stop or drivers' WC rush). Solair is JUST a compressor replacement in a no essential or safety critical vehicle part. Out goes the mechanical, in comes the thermal compressor. All the rest of the AC system is not changed or modified.

Our business strategy is to start with city buses, move to trucks and then to cars/vans. At any moment that an vehicle manufacturer decides to join the change from a mechanical to a thermal compressor, he will be most welcome and a special small team will be set up (costs covered by the vehicle manufacturer) to make the link between Solair general strategy and this vehicle manufacturer desires. Solair will try to best accommodate the development of this vehicle manufacturer fitting solution (in trucks and buses, space is not a critical issue, but in small vehicles, the fitting will be a challenge since different models will have different requirements) without compromising or delaying the initial program proposed here.

To develop Solair Prototype, expertise in mobile AC, manufacturing, vehicle installation and procedures will be needed. So a team building process will start on day one and will go on as long as necessary. Experts and driven people can be found in US Universities, Research centers and Startup communities in social nets, hubs, accelerators, etc.

Cities mayors are sensitive to public perceptions and demands. By taking a step where comfort, fuel savings, GHG and pollution, cost reduction is archived by a simple change of compressors in the city buses fleet, mayors will be willing to support such change.
This will bring vast media coverage, access to capital to implement a manufacturing facility.

Once this is done, this initial production facility will grow allowing the company to offer thermal compressor for vehicle owners of first selected models (chosen among the model of vehicles manufacturers that adopt Solair for its new models), so a Brand will have new and sold models fitted with our thermal compressor, gaining on vehicles owners intangibles values.

City mayors will also be able to help with the red tape, key legal, authorizations and regulatory agencies, so Solair automotive can be readily adopted by vehicles manufacturers.

  1. Team and R&D Selection - Selection of a CTO, COO, CFO, etc. are the first critical steps. Once the initial team is build, the detailed strategy within the initial strategy proposed here will be developed. The team will select the R&D center/facility in one of the southern US states to support Solair automotive prototype development for city buses.
  2. Prototype development - Done with the close participation of at least one US bus chassis manufacturer, Solair automotive for city buses will be developed. In case, at least one chassis manufacturer decided not to join the program, Solair will work first on the chassis most in use by the 10 partner cities buses fleet. One chassis will be bought and used in the R&D facility selected to develop the prototype (in case no chassis/body manufacturer want to participate in the Solair prototype development, it will be done solo with MIT/Solve/CoLab and Solair team working together!).
  3. Initial manufacturing of Solair prototype - Solair will hire for the first 1,000 units one or several SME to manufacture the compressor units with close and total Solair supervision.
    Parts and equipment needs will be bought, rented or borough, depending on best solution and long time strategy.
  4. Solair buses/truck installation partnership - Since Solair business is to produce the thermal compressors, the changing of the mechanical by the Solair thermal will be done by selected and trained AC repair shops. We will start with the interested shops located in the 10 partner cities, creating jobs and increased local business revenues.
  5. Development of a crowdfunding campaign among cities (all in the US that would like to join to get a cleaner, less pollutant and cost city bus fleet) interested in the changing the mechanical by a thermal compressor in its city buses fleet. This campaign will support the development of the first manufacturing plant in the US.

Who will take these actions?

1) Team building - Solve/MIT/CoLab team will help Solair in the selection of the initial team. Once the team is selected, join meetings will decided the best final design for the system. Once it is done, each member will be signed a duty in the following areas

  • Governments and regulatory agencies relations 
  • Chassis/Body manufacturer relations
  • Public/Marketing relations
  • Part suppliers chain
  • Solution Installation AC/R repair shops partnerships relations
  • Solution installation Training 
  • Vehicles manufacturers relations

2) CTO - Prototype development will be done using MIT/Solve partners workshops. Attention will be made so the development is done with the initial target of 1,000 thermal compressor units in mind.

The steps are

  1. Make first model in CAD/CAM,
  2. Make the prototype, 
  3. Installed in a city bus (parked somewhere at MIT/SOLVE),
  4. Runs it for a while (abusing of it, hard, trying to destroy the thing!),
  5. Fix bugs, problems, make adjustments, make new version of prototype,
  6. Make 5 units, 3 to city buses, 2 for intra-city buses (long distance buses, different requirements, demands on it), 
  7. Test on real life conditions for 4-6 months, collect data, solve problems, bugs, fine tune it
  8. Initial manufacturing of the first commercial version.

3) COO - Will propose to the team an strategy for

  • Governments and regulatory agencies relations 
  • Chassis/Body manufacturer relations
  • Public/Marketing relations
  • Part suppliers chain relations
  • Solution Installation AC/R repair shops partnerships relations
  • Solution installation Training 
  • Vehicles manufacturers relations

4) Solair first manufacturing plant - this is a critical and very important aspect of a manufacturing startup. So, a standard proposal will be presented to all States/City authorities in the selected regions(s) asking for the submission of a site/incentives/financial/grants proposals. MIT/Solve/CoLab assistance will be part in this critical and vital step.

Target geography

Due to the fact that heat is a more pressing issue in the south of the USA, we will start with the south States large Cities, such as Los Angeles, San Diego, Miami, Dallas, Huston, Albuquerque, Las Vegas, New Orleans, etc.

This cities will gives us an National coverage and exposure, large city buses fleets and access to public support, financial and team/experts access.

Initial talks and a proposal submission will be made to all selected southern US cities with large city buses fleet and a repair/service AC workshop partners in place.

Once the contract has been signed and the initial capital is ready for the manufacturing of first 1,000 units, development, production, training, installation and monitoring will start.

Monitoring is a key step once the solution is up and running, so Solair can collect data on performance, durability, operational and maintenance costs to be used in future developments for other markets (trucks, cars, vans, etc.).

What do you expect are the costs associated with piloting and implementing the solution, and what is your business model?

This will depend on the number of cities buses and cites that join in. Taking 100 buses per city and 10 largest cities in the south of the USA (totaling 1,000 buses initial test).

An estimated cost of US$100,000.00 for prototype development.
An estimated cost of US$300,000.00 for team building and traveling expenditures.
An estimated cost of US$600.00/compressor totals US$600,000.00
Brigs the grand total to US$1,000,000.00

Divided this cost among the 10 partners cities, each will have to contribute with US$100,000.00.

Once this prototype and initial trails phase is done, increase production will be implemented to replace in all US cities buses fleet the mechanical compressor by Solair thermal compressor.

At this point, the startup will be well funded and in the blue.


The fastest way to develop Solair prototype is to do it with a city bus chassis/body manufacturer in close partnership. 

There are the steps

1.     Exhaust gases flow and diversion - Thermal compressor switching on/off will be done by the temperature band (amplitude, min/max). Exhaust gases can not keep always passing by the thermal compressor since even that the flow of the refrigerant gas could be stopped, the rest of the refrigerant gas left inside of the compressor would suffer chemical degradation, compromising the rest of the system. A double exhaust tailpipe section will be needed to divert the exhaust gases flow to the compressor and off when the compressor is off. See first image on top.

2.     Controls - Compressor switching on/off will be done by the temperature band (amplitude, min/max), refrigerant gas storage levels and pressure monitoring. The controls will be placed in a touch screen solution close to the driver (body manufacturer discussion and suggestions).

3.     Refrigerant gas storage - for city buses, an storage capacity of two (or less, depends on operational drivers procedures) hour shall be sufficient. This will allow for driver have lunch or go to the wc with the bus engine off but the AC on, keeping the bus cool under the sun.

4) Refrigeration Unit and tubing - Each bus model (size) will need a number of refrigeration units (to better distribute the cool) and tubing to deliver and collect the refrigeration gas, plus a wind curtains at entrance and exit doors to avoid cool loses. If the bus has already an AC system installed, this will not be needed.

Refrigeration units can be bought from a on market manufacturer, tubing is standard AC tubing. The compressor prototype will be CAD developed and CNC manufacturer in MIT or Solve partners workshops. Assemble of the solution will be done at MIT, Solve partner workshop or city bus chassis/body manufacturer workshop.
Monitoring will be done by Solair team/staff
Training will be done by Solair team/Staff.

Related solutions

Fuel consumption reduction is a critical and vital aspect in GHG, pollution and cost reductions.
Some proposals address this issue, such as




This is a Canadian Government site enplaning the fuel consumption increase in vehicles using AC

This a paper published by the NREL "Impact of Vehicle Air-Conditioning on Fuel Economy, ..."

This article put the numbers on how much fuel AC in the US uses on yearly basis! "The study showed that the US uses 7.1 billion gallons (27 billion liters) of gasoline every year for air conditioning vehicles, equivalent to 6% of domestic petroleum consumption, or 10% of US imported crude oil."

São Paulo City report on public transport and mobility plan (in Portuguese)

Los Angeles Metro


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Solution Summary
Thermal compressor for mobile and fixed HVACR systems
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By:  Solair
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