Thermofluidics’ core Non-Inertive-Feedback Thermofluidic Engine (NIFTE) is a patented heat engine technology: It converts heat at a higher temperature into mechanical work (forced motion), and heat at a lower temperature.
What is new and different about the NIFTE?
The NIFTE is distinguished from other heat engines in that it is capable of using relatively small temperature differences, as low as 30ºC between its heat source (heat input) and heat sink (heat output), and it does this without mechanical moving parts. Instead, it has liquid pistons which move up and down in response to pressure variations, caused by boiling and condensation.
As it has liquid pistons, the NIFTE is particularly well suited to pumping and compressing fluids. It is also well suited to pumping, because the pumped fluid can simultaneous provide either the heat supply, or the heat sink.
How the NIFTE works
- Two vertical cylinders (1,2) are cojoined at top (3) and bottom, via a throttle valve/restriction (4)
- Grey liquid boils in evaporator (5), pressurising the system with vapour (colourless).
- When the pressure exceeds the “discharge
- pressure, fluid coupled to working fluid in fluidic transmission block (7) is discharged.
- A discharge check (non-return) valve (6) may be used to arrange a uni-directional flow.
- As fluid is discharged, the grey liquid in cylinder 2 descends.
- When the liquid-vapour interface in cylinder 2 is below the vapour-liquid interface in cylinder 1, grey liquid flows from cylinder 1 to cylinder 2 due to gravity, with a time delay due to valve/restriction 4. It is important that the fluid’s inertia doesn’t play a significant role in generating this time delay, hence the term “Non-Inertive-Feedback”.
- When the liquid-vapour interface in cylinder 1 has descended into condenser (8), vapour condenses, de-pressurising the system.
- When the pressure falls below the “suction
- pressure, fluid coupled to working fluid in fluidic transmission block (7) is drawn into the system.
- A suction check (non-return) valve (9) may be used to arrange a uni-directional flow.
- As fluid is drawn in, the grey liquid in cylinder 2 rises.
- When the liquid-vapour interface in cylinder 2 is above the vapour-liquid interface in cylinder 1, grey liquid flows from cylinder 2 to cylinder 1.
This process is repeated periodically, and results in a net-pumping effect. In some applications, the working fluid and the pumped medium can be the same type of fluid.
Download a film of an early NIFTE prototype made from glass
You can see a short film of an early NIFTE prototype by clicking here. To see another short film of a NIFTE with an output accumulator (to steady the pulsed output flow), click here
What is special about being able to use low grade (low-temperature) heat?
Unlike high grade energy forms, such as electricity, fuels, or heat at high temperatures (e.g. from burning fuels), low grade (low-temperature) heat is abundantly available as a waste product from other processes, or at relatively low cost from sunshine. All of the costs associated with low grade heat engines are up-front (capital) costs. Therefore, the engineering focus is more on minimising these costs than minimising heat consumption.
The thermodynamics of low grade heat engines
Although efficiency is not of primary importance (as low-grade heat is generally renewable and free), it is of secondary importance, as the heat throughput of a device dictates size, and therefore cost. According to the second law of thermodynamics, the maximum theoretical efficiency of a low grade heat engine will always be lower than for a high grade heat engine.
The way input heat ends up in typical heat engines is plotted against the temperature difference (Delta T), between heat source and heat sink in the plot below.
The blue area represents heat that can never be converted into useful work, due to the second law of thermodynamics. For low values of Delta T, this is by far the biggest component of the overall losses. The other colours represent other types of loss, which are theoretically avoidable, though practically always present to some extent. These are described below. The white area represents the useful work output.
Why a two-phase (boiling and condensing) thermodynamic cycle?
In order to heat a gas or a liquid, a large “excess” temperature is required, to force heat energy into the fluid. In other words, a solid surface in contact with the fluid must be at a significantly higher temperature than the bulk of the fluid to drive heat into it. However, if liquid boils at the surface, a much smaller excess temperature is required. Similarly, if vapour condenses at a surface, a much smaller excess temperature is required in the vapour, in order to remove heat from it.
The excess temperatures required to drive heat flows in heat engines do not change much with the temperature difference (Delta T), between the heat source and heat sink. Therefore, they tend to represent a much greater proportion of Delta T for a low heat engine Delta T, than for a high Delta T heat engine.
If the second law loss (blue area in the plot above) is removed, and the remaining colours spread evenly across the chart area, the result is the plot below. The losses (coloured) are now referred to as “exergy” losses – losses once second law losses have already been accounted for. These are the losses which can always be reduced by improving design.
It can be seen that the heat exchange loss always represents a much greater proportion of the overall exergy loss in a low Delta T heat engine, than for a high Delta T heat engine. Therefore, using phase change (boiling and condensing) processes in order to keep excess temperatures low is essential to achieving good performance at reasonable cost.
The economics of low grade heat as an energy source
In order to minimise up-front (capital) costs, there are a number of issues of importance:
- The heat throughput of the heat exchangers (and solar collectors, if applicable), which determines their mass and complexity.
- The choice of construction materials, and the mass of each required.
- The production processes employed.
Thermofluidics has expended considerable efforts finding optimal solutions in all of these areas, without compromising lifetime or performance.
Performance of the NIFTE
The performance of the NIFTE can be measured in four ways:
1. The flow rate of fluid it can pump.
2. The head (height or pressure) it can pump against.
3. The amount of heat consumed to do this.
4. The temperature difference (Delta T) between heat source and heat sink.
The last two of these areas can be traded off against each other by adjusting the valve/restriction (4, in the “how the NIFTE works” schematic).
Progress to date
The plot below illustrates progress made in these areas. The valve/restriction has been set to minimise heat input, rather than prioritise low Delta T, which is about 50ºC externally, with an excess temperature of around 15ºC in the evaporator, and 20ºC in the condenser. The x-axis shows the flow that the NIFTE can pump in litres per hour, per kilowatt hour of heat input. The y-axis shows the head (height) in metres that the NIFTE can pump this flow rate to. The dots are actual (measured) data points and the curves are fits generated by computer models.
The black curve (“Initial”) shows the performance of an early glass prototype, similar to the one that can be viewed in the film referred to above. The red curve (proven) shows the performance of a more recent prototype, made largely from plastic.
To date, Thermofluidics has prioritised increasing flow rate, over pumping large heads/pressures. This is because the majority of the early “low-hanging-fruit” applications require quite high flows but fairly insignificant heads. The green, blue and yellow points that lie along the x-axis show improvements in flow at zero head, made largely through geometry (size and shape) improvements. With existing materials and no further development, we project the dashed yellow curve for our next-generation field trial prototype design.
Thermofluidics’ progress in improving flow rate, and flow rate per heat input at low head (0 to 1m), is shown in the plots below. The red lines indicate improvements to early electrically heated prototypes. The yellow lines indicate improvements to prototypes heated by a vapour thermosiphon (or gravity return heat pipe), which can be used to transfer heat out of awkwardly shaped, or large objects such as a solar thermal collector. The blue lines indicate two separate phases of development in which a NIFTE pump was embedded into a gas-fired condensing boiler as a circulation pump.
In addition to instantaneous performance, lifetime, reliability, and ability to start and stop automatically with a minimum of intervention are all important measures of progress. Due to its general lack of moving parts, and insensitivity to small changes in materials properties and geometry with time, the NIFTE should fare well on all of these counts. The field trials will be an important step in verifying this, and addressing any problems that arise.
Thermofluidics’ intellectual property
The description above is a vastly simplified explanation of the working principles of the NIFTE. In practice, the ratios of sizes and shapes (geometry), the choice of construction materials (materials) and the design of the heat exchangers (5 and 8), are all critical to obtaining good performance, as well as lowering cost. Thermofluidics has generated, and is continually generating new Intellectual Property in these areas of expertise.
In addition to the NIFTE, Thermofluidics has new Intellectual Property associated with our proprietary hydraulic ram, an adaptation of the famous Mongolfier hydraulic ram, for lifting water from deep wells and boreholes. We are in the process of filing for patent protection for this technology. Further details will be posted on our hydraulic ram page in due course.
- Absorption and vapour compression refrigeration, air-conditioning and heat-pumps.
- Vapour compression and multi-effect salt water desalination.
- Vapour compression air-dehumidification.
In the long term we also hope to increase the frequency of NIFTE devices to a level at which they will be suited to power generation and electrically powered thermodynamic applications.