Various applications could benefit from a new pumping technology, ranging from domestic central heating circulator pumps to irrigation systems in developing countries. Jon Severn reports on the Non-Inertive Feedback Thermofluidic Engine (NIFTE) that is currently under development.
Pumps can be relatively large consumers of energy, which is a problem when energy costs are high or there is a need to operate a pump in a location where there is no grid-supplied energy. Traditionally the answer has been to suffer high energy bills in the former situation and, for off-grid applications, use technologies such as wind turbines or photovoltaic cells that have a high capital cost and low efficiency.
However, there is now an alternative technology being developed that is simple and costs little to manufacture, yet it is highly effective and can operate using low-grade heat energy - such as waste heat or solar energy. This type of heat pump has been termed the Non-Inertive Feedback Thermofluidic Engine (NIFTE).
The NIFTE technology and pumps are being developed by Dr Tom Smith and Dr Christos Markides, who have formed a company called Thermofluidics. Thus far they have been supported by competition prize money and government grants, and they are currently seeking further funding to continue the development.
NIFTE heat engines are described as being analogous to RC (resistor-capacitor) feedback electronic oscillators that do not depend on inductors to generate and sustain oscillations. By following similar design principles, the researchers have developed NIFTEs that require only small amounts of thermal or viscous dissipation, and can therefore sustain large pressure amplitude oscillations with high efficiency. Furthermore, the NIFTEs benefit from a high power density.
Being capable of operating across low temperature differences means that NIFTEs can utilise low-grade heat (such as solar thermal energy) or waste heat. This is in contrast to most other types of heat engine that cannot operate on such low temperature differences. The low-grade heat on which the NIFTE pumps operate is both abundantly available and cheap to obtain, which makes the actual efficiency of the pump less important, as their feasibility is determined by their efficiency per unit production cost. Fortunately NIFTE pumps can be manufactured from low-cost materials such as plastics and polymer foams, using low-cost production techniques including injection moulding and extrusion. Only a smalll number of precision moving parts are required, typically just a pair of non-return valves.
How the NIFTE pump works
The NIFTE has two vertical cylinders (1 and 2 ) that are connected at the top (3). Another connection at the bottom incorporates a restriction or throttle (4). The system is part-filled with a working fluid (shown grey), which can be water or another fluid, depending on the temperature of the heat source and heat sink available, as well a the application requirements. For example, in an irrigation pump the working fluid will be water, the same as the fluid being conveyed; in a closed-circuit heat pump application, the working fluid could well be something else.
In operation, the top section of the left-hand cylinder (the 'displacer' cylinder) is heated until the liquid boils, whereupon the pressure of the resultant vapour (colourless) forces liquid in the right-hand cylinder (the 'power' cylinder) down into the fluidic transmission block (7). Because this liquid is coupled to the pumped medium, the pumped medium is forced out of the pump (8).
This discharge results in the liquid-vapour interface in the right-hand cylinder falling below the liquid/vapour interface in the left-hand cylinder. Gravity then causes liquid to move from the left-hand cylinder to the right-hand cylinder via the throttle (4). As the liquid/vapour interface in the left-hand cylinder then moves lower into the cold region (9), vapour condenses, the pressure drops, and replenishing liquid (10) is then sucked into the right-hand cylinder via the fluidic transmission block.
The cycle repeats in an oscillatory fashion, with a net pumping effect. Although the direct output is pulsed, an accumulator can be used to smooth the output.
Note that in the majority of applications the pumped medium provides either the heat source or (more often) the heat sink, and is therefore circulated around the hot heat exchanger (6) or the cold heat exchanger (9), respectively.
Given their mechanical simplicity, low energy consumption and potentially low purchase cost, NIFTE pumps could be applicable for a wide variety of tasks. These range from irrigation and pond pumps to central heating circulating pumps and power showers. Geothermal pumping is also an attractive application, and there are industrial applications relating to hot water circulation and steam cleaning.
So far Smith and Markides have focussed on two applications, one commercial and the other philanthropic. The commercial application is a central heating circulation pump and the philanthropic application is an irrigation pump.
In their laboratory within the Cambridge University Engineering Department the researchers adapted a 19kW domestic heating boiler (condensing combination type) and progressively developed the NIFTE pump from an initial output of 400 litres/hour in September 2007 to 1100 litres/hour in May 2008. In this system heat is taken from the existing burner flame by means of a simple heat exchanger, with waste heat being rejected into the hot water that the pump is circulating through the radiators. The pumped water is the same as the NIFTE working fluid. Note that the pump is self-starting, with a small number of oscillations required before the pump achieves its normal operating state. Depending on the design of the pump and the application, the start-up time can be set to be anything from a few seconds to several minutes. Likewise, the pump stops by itself when heat is removed from the hot heat exchanger, with the run-down time depending on the pump design and application.
In common with almost all pumps, the NIFTE achieves its maximum flow rate at zero head, though the associated efficiency is low. As the head is increased, and the pump works against a greater load, the flow rate decreases until eventually the pump stalls and flow ceases.
For typical one-bedroom apartments/flats with one bathroom, the maximum heating capacity for the central heating system varies from 10 to 20 kW. Electric circulator pumps in such systems are required to pump between 500 and 800 litres/hour at dynamic heads of 0.5 to 1.5m. Systems installed in typical two- or three-bedroom semi-detached houses with one bathroom and an en-suite deliver 15 to 25 kW and incorporate circulator pumps capable of delivering 600 to 1000 litres/hour at heads of 0.5-3.5m. Since circulators operate within a closed loop that does not exhibit a static head loss, the quoted heads are due to viscosity in the piping rather than any height differences. The demonstration NIFTE installed on a 19 kW boiler has been shown to flow 1100 litres/hour at nominally zero head, 1000 litres/hour at 1 m head, 900 litres/hour at 2m head and 800 litres/hour at 3m head.
The researchers have established that the flow rate achievable by a NIFTE pump scales with the heat available and the diameter of the power cylinder (2 in Fig.2), until the capacity of the heat exchangers to exchange heat at the temperature differences set by the application is saturated.
Other NIFTE pumps have been built, one of which stalled at heads of approximately 4m and another that had a stalling head that is estimated to be around 20m.
In this application heat is taken from a solar hot water collector by means of a heat pipe or thermosiphon and supplied to the hot heat exchanger of the NIFTE. The NIFTE rejects its waste heat into the water that is being pumped, as with the central heating circulator application.
According to Smith and Markides, the potentially large market for solar-powered pumps is currently restricted by the prohibitive cost of photovoltaic (PV) cells, hence only small and diverse applications benefit from solar pumps. However, the researchers say that studies by the United Nations and other non-governmental organisations have estimated the need for 30 million solar-powered water pumping systems around the world, and the researchers believe that the market for low flow-rate agricultural applications alone is worth over £2billion (around US$3billion or EUR2.5billion).
NIFTE pumps are said to be particularly well suited to pressurising surface water for relatively low flow-rate applications such as drip irrigation. Drip irrigation consumes up to 70 per cent less water than conventional irrigation practices. The current market for drip irrigation is thought to be worth approximately £300million (around US$470million or EUR370million), mostly in the USA, and is growing at 15 to 20 per cent per annum due to increasing use in Southern Asia.
In the solar water pump application, the NIFTE pump would be thermally coupled to an array of solar hot water collectors. Compared with conventional PV-powered electric pumps, the NIFTE arrangement is said to offer a number of significant advantages, such as a lack of moving parts, low capital cost and high power density.
Smith and Markides have already built and tested a rudimentary solar-powered NIFTE pump in which the hot heat exchanger takes its heat from a thermosiphon capable of modelling the solar collectors. This prototype has not yet been investigated extensively or optimised, but already flow rates of 620 litres/hour have been achieved with 600 W of heat at zero head and 480litres/hour with 600 W of heat at 1m head. The test apparatus is capable of providing up to 3kW of heat, simulating approximately 6 m2 of typical flat plate solar collectors. Given the rate of progress to date, the researchers are confident that the NIFTE solar pump will be capable of flows of about 750 to 1000 litres/hour and 5 to 10m heads.
Aside from the two applications outlined above, there are numerous other potential applications for NIFTE technology. One area has been termed thermally-powered pressure boosters, with examples being steam cleaners, power showers and espresso coffee machines. In these applications, the NIFTE pump could replace electrical pumps and heaters, offering the advantages of greater reliability and lower capital and operating costs.
Another area of interest is absorption refrigerators. Compared with vapour compression refrigeration systems, there is no need for a mechanical compressor and a NIFTE-powered absorption refrigerator could operate without mains electricity. Solar-powered NIFTE absorption refrigerators could, for example, be used in developing countries to maintain the temperature of vaccines.
Smith and Markides' company Thermofluidics is currently seeking to grant licenses, provide consulting services and/or establish development partnerships and collaborations with industrial partners with a significant market share in each application area to help bring the technology to market. The company does not intend to become a manufacturer, but the potential advantages of this technology are likely to be attractive to a spread of manufacturers currently relying on conventional technologies.