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References

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Renewable Energy Systems

Scientific Articles

(Abstracts and comments are given further down the page.)

[1] “A Heat Engine and Heat Pump based on the Bernoulli Effect”, 18 pp, Sunoba Pty Ltd (2006).

[2] “The Bernoulli Evaporation Turbine and Bernoulli Condensation Heat Pump”, 20 pp, Sunoba Pty Ltd (2006).

[3] “Thermodynamic Model for a Two-Stroke Evaporation Engine”, Proc ANZSES Conf, Canberra (2006).

[4] “An Evaporation Heat Engine and Condensation Heat Pump”, ANZIAM J, Vol 49 (2008), 503-524.

[5] “Experimental Results for a Heat Engine Powered by Evaporative Cooling of Hot Air at Reduced Pressure”, Proc ANZSES Conf, Sydney (2008).

[6] “Confidential”, 14 pp plus 9 diagrams, Sunoba Pty Ltd (2009).

[7] “Annual Output of a New Solar Heat Engine”, Proc AuSES Conf, Canberra (2010).

[8] “The Expansion-Cycle Evaporation Turbine”, J Eng Gas Turbines and Power, to appear (2011).

[9] “Output of the Evaporation Engine (Sloping Canopy)”, Proc 2011 Solar World Congress, Kassel (2011).

[10] “ECET boost to solar-hybrid gas turbines”, Proc AuSES Conf, Sydney (2011).

Technical Reports

2007-1 Droplet evaporation tests at constant volume

2007-2 Leakage/friction tests for the experimental BEE

2008-1 The experimental BEE – design, construction and first trials

2008-2 The experimental BEE – further trials

2008-3 The experimental BEE – successful trials

2009-1 Simulation of the BEE with Scotch Yoke

2009-2 Numerical simulation of droplet evaporation

2009-3 BEE and BDE technology overview

2009-4 Conceptual design for a 6 kg clothes dryer that incorporates the BDE

2009-5 Simulation of the BEE with crosshead

2009-6 Prototype BEE – requirements and concept

2010-1 Simulation of incomplete evaporation for BEE re-compression; Tinlet = 200°C

2010-2 Passive solar heating under a transparent canopy

2010-3 Optimal flow-rate for the passive solar BEE

2010-4 The Wellington simulations

2010-5 The Wellington simulations – sloping canopy

2011-1 Beyond-Carnot heat pump?

2011-2 COP of the continuous-flow condensation heat pump with losses

2012-1 Beyond-Carnot heat pump – further details

[1] N.G. Barton, “A Heat Engine and Heat Pump based on the Bernoulli Effect”, 18 pp, Sunoba Pty Ltd (2006).

This paper investigates a device based on the Bernoulli effect for a compressible gas, which can be configured either as a heat engine or heat pump. The Bernoulli effect shows there is a drop in pressure, temperature and density when gas accelerates isentropically in a narrowing section of a duct. If heat is removed from the gas in the high--speed section before the flow is transferred isentropically to the outlet, there will be surplus pressure to drive a turbine once the gas has been slowed. That is, the device can act as a heat engine. Conversely when heat is transferred to the high-speed section, the device can act as a heat pump.

Presented herein is a one-dimensional thermodynamic model to predict the performance of the proposed heat engine and heat pump. Also presented are budgets for enthalpy and entropy increments, which confirm that the engine is in agreement with the second law of thermodynamics. The devices operate with Carnot efficiency and Carnot coefficient of performance provided the amount of heat transferred is small.

Comment: This unpublished note describes a theoretical manifestation of the low-pressure gas cycle underpinning the evaporation engine, in which the Bernoulli effect provides the necessary low-pressure zone. The Bernoulli devices that are analysed require a near-to-sonic-speed heat exchanger, which everyone agrees is technically impossible. Despite the practical impossibility of the devices described, the article is useful as a backgrounder.

Available on request.

[2] N.G. Barton, “The Bernoulli Evaporation Turbine and Bernoulli Condensation Heat Pump”, 20 pp, Sunoba Pty Ltd (2006).

The Bernoulli effect for a compressible gas shows there will be a drop in pressure, temperature and density when the gas accelerates in a narrowing section of a duct. If the inlet air is hot and dry, spray evaporation of water in the high-speed section leads to increased pressure and density and decreased temperature and air speed. If the air is then slowed isentropically without flow separation, the outlet pressure will be greater than the inlet pressure, thus enabling power off-take through a turbine. That is the principle of the Bernoulli evaporation turbine.

If the inlet air is cool and moist, the Bernoulli effect leads to rapid condensation of microscopic water droplets in the high-speed section and release of latent heat. This causes decreased pressure and density and increased temperature and flow speed. If the droplets are collected and thereby prevented from re-evaporation as the flow is isentropically slowed without flow separation, the outlet temperature will be greater than the inlet temperature. It is necessary also to provide power to extract the air from the flow device. That is the principle of the Bernoulli condensation heat pump.

This paper presents a thermodynamic model for the proposed engine/heat pump. Illustrative results are also presented. Although the engine operates with low efficiency, it theoretically would provide energy in a sustainable way from inputs that are cheap and readily available.

Comment: This unpublished note is an extension of [1] for the case where evaporation or condensation provides heat transfer in the low-pressure zone. Can these Bernoulli devices be constructed? Opinions differ. Our view is that the devices are probably impossible, but not certainly so, and with a clear-cut answer requiring brilliant and painstaking CFD design work. If the Bernoulli devices do in fact work, the implication is that expansion and re-compression take place without mechanical intervention -- a striking result. Moreover, the Bernoulli devices would involve continuous flow, not a batch process as in reciprocating engines. The results underestimate the theoretically possible performance of the devices, since evaporation during re-compression is neglected for the heat engine as is condensation during expansion for the heat pump.

Available on request.

[3] N.G. Barton, “Thermodynamic Model for a Two-Stroke Evaporation Engine”, in Proc Australian and New Zealand Solar Energy Society Conference, Canberra, 13-15 September 2006.

This paper investigates a heat engine designed to produce power and a cool moist exhaust from evaporative cooling of hot dry air at reduced pressure. Theoretically, the thermodynamic cycle underpinning the engine is a low-pressure Brayton cycle, which can be manifested in various ways. Of these alternatives, the simplest from an engineering standpoint is a two-stroke reciprocating piston arrangement.

A thermodynamic model for the engine is presented and applied to the case where the inlet air is pre-heated. Under suitable weather conditions, and assuming evaporation to saturation at constant volume in the low-pressure section and no further evaporation, the theoretical result is that the engine converts approximately 4-5% of energy collected at 30-40^oC above ambient into electrical or mechanical power. Whilst this efficiency is not high, the inputs will be inexpensive since the requisite pre-heating can be accomplished by passive solar methods, and hot air is both the heat transfer medium and the working gas. The engine does not require heat exchangers or condensers as with Rankine cycle engines. An estimate is also made for the nett power produced with pre-heating by the engine per hectare per year under typical conditions in inland Australia. Assuming pre-heating by 30^oC, the estimate is 417 MWhr/(Ha.yr).

Comment: This unrefereed conference paper contains early theoretical results on the evaporation engine. Evaporation during compression, which is a significant source of available work, is not included.

Available on request.

[4] N.G. Barton, “An Evaporation Heat Engine and Condensation Heat Pump”, ANZIAM J, Vol 49 (2008), 503-524.

This paper presents a thermodynamic model for a heat engine based on evaporative cooling of unsaturated air at reduced pressure. Also analysed is a related heat pump based on condensation of water vapour in moist air at reduced pressure. These devices operate as two-stroke reciprocating engines, which are their simplest possible embodiments.

The mathematical models for the two devices are based on conservation of mass for both air and water vapour, ideal gas laws, constant specific heats, and, as appropriate, either constant entropy processes or cooling/heating by evaporation/condensation. Both models take the form of coupled algebraic systems in six variables, which require numerical solution for certain stages of the cycle.

The specific work output of the heat engine increases as the inlet air becomes hotter and as the expansion ratio of the engine increases. The engine provides evaporative cooling of air from inlet to outlet. The heat pump has a good coefficient of performance, which decreases as the expansion ratio increases. The heat pump also has the effect of drying the air from inlet to outlet, producing distilled water as a by-product.

Comment: This refereed journal paper contains full thermodynamic models for the evaporation engine and condensation heat pump based on two-stroke reciprocating mechanisms. Evaporation during re-compression is included in the evaporation engine, as is condensation during expansion in the condensation heat pump.

Available on request.

[According to copyright agreement, a PDF copy of the paper can be provided to researchers for their personal use, provided that they do not make the file available to anyone else. For further information, please visit the publisher’s website.]

[5] N.G. Barton, “Experimental Results for a Heat Engine Powered by Evaporative Cooling of Hot Air at Reduced Pressure”, in Proc Australian and New Zealand Solar Energy Society Conference, Sydney, 26-28 November 2008.

This paper describes the design, construction and first trials for an experimental heat engine based on a novel thermodynamic cycle. Design work on the engine started in mid 2007 and construction was completed in April 2008. After various modifications, successful trials first occurred in August 2008.

The thermodynamic cycle is thought to be completely new and was developed during an investigation into power generation based on passive solar heat collection. The cycle involves intake of hot unsaturated air, expansion, evaporative cooling at constant volume, re-compression with further evaporation, and exhaust of cool saturated air. In the experimental engine, this cycle is realised with a piston-in-cylinder mechanism.

Successful trials of the engine have occurred for inlet air temperatures as low as 70°C, thereby offering support to the overall concept and the thermodynamic analysis. Weaknesses in the design concept have been identified and recommendations made for improved performance. Immediate future plans involve development of a combined-cycle engine in which the new evaporation cycle is integrated with the Brayton gas power cycle. The target market for this development is the provision of Combined Heat and Power for medium-sized buildings. The original goal of power generation directly from hot air remains under consideration for commercial development.

Comment: This refereed conference paper contains details of the experimental evaporation engine and its first successful trials.

Available on request.

[6] N.G. Barton, “Confidential”, 14 pp plus 9 diagrams.

Comment: This manuscript has not yet been submitted for publication. It is a revised version of the heat pump application in article [2], in which condensation during acceleration and losses during deceleration are explicitly taken into account.

This article will be held confidential until further notice.

[7] N.G. Barton, “Annual Output of a New Solar Heat Engine”, in Proc. Australian Solar Energy Society Conference, Canberra, 1-3 December 2010.

This work describes a year-long simulation of a new heat engine at a suitable inland site. In this concept, air is heated passively under a transparent insulated canopy and then presented directly to the inlet valves of the engine. The thermodynamic cycle of the engine is based on evaporative cooling of hot dry air at reduced pressure. Air is both the heat transfer fluid and working gas of the engine. Heat exchangers are not needed, and the engine will be multi-cylinder, large, slow-revving and lightly stressed.

The canopy heating model incorporates reflection-absorption-transmission of sunlight and infrared radiation. Other features of the model include heat diffusion through a convection-suppression region adjacent to the glass cover of the canopy, and convective heat transfer from canopy to atmosphere. The canopy model is coupled to a thermodynamic model of the heat engine, and the mass flow-rate at each time interval is calculated to give optimal power output subject to canopy heat losses.

With canopy losses included but not engine losses, the peak power output from the sample of 51 days that was simulated was 89 W/m^2. The annual output was estimated to be 104 kWhr/(m^2.yr). Engine losses are expected to reduce these estimates, which are for a horizontal collection surface, by 25-30%.

Comment: This peer-reviewed conference paper contains details of the year-long simulation of the performance of the evaporation engine at Wellington NSW.

Available on request.

[8] N.G. Barton, “The Expansion-Cycle Evaporation Turbine”, J Engineering for Gas Turbines and Power, to appear (2011).

This paper investigates a continuous-flow heat engine based on evaporative cooling of hot air at reduced pressure. In this device, hot air is expanded in an expansion turbine, spray-cooled to saturation and re-compressed to ambient pressure in several stages with evaporative cooling between each stage. More work is available in expansion than is required during re-compression, so the device is a heat engine. The device provides a relatively cheap way to boost the power output of open-cycle gas turbines.

The principal assumptions for the theoretical model developed herein are that air and water vapour are regarded as ideal gases with constant specific heat capacities. In the absence of losses associated with expansion and compression, the engine produces more power as the inlet temperature and the pressure ratio increase. The effects of irreversibilities are subsequently included in the expansion and compression stages, with realistic values used for the adiabatic efficiencies of turbine and fans. Purification and injection of water are also considered in the overall energy budget.

As a typical result for the new engine, if the inlet air is the exhaust of a 56 MW open-cycle gas turbine, the adiabatic efficiencies of turbine and fan are 0.9, the pressure ratio is 6.5 and there is four-stage re-compression, then the power output is 20.5% that of the gas turbine. The power output is sensitive to the adiabatic efficiencies of turbine and fans.”

Comment: This paper contains a full theoretical analysis together with the case study described in the ECET web page. Accepted for publication: 16 July 2011. Expected publication date: late 2011.

Details can be made available now to potential licensees under Non-Disclosure Agreement.

[9] N.G. Barton, “Output of the Evaporation Engine (Sloping Canopy)”, in Proc. 2011 Solar World Congress, Kassel, 28 August – 1 September 2011.

The paper describes new simulations for a heat engine powered by passive solar heat collection under a transparent insulated canopy. The engine’s thermodynamic cycle is based on evaporative cooling of hot air at reduced pressure (Barton, 2008a). An experimental version of this engine has been successfully tested (Barton, 2008b) and many other aspects have been studied, including limitations to the engine speed as a result of incomplete evaporation during re-compression.

Recently Barton (2010a) simulated the annual output of the evaporation engine at a suitable site with passive solar heat collection under a horizontal canopy. The present work contains new simulations based on a canopy that slopes at the latitude angle. The air flow-rate under the canopy was chosen to optimise the power output over half-hourly intervals. The power output was aggregated over the day and the whole procedure repeated for a representative sample of days.

In the case of the horizontal canopy, Barton (2010a) showed that the daily power output can be approximated using a linear function of the total daily insolation. For the sloping canopy, however, additional modeling assumptions are required to relate sample results to both the time of year and the total daily insolation. It is found that the sloping canopy leads to more power output over the year and in a more evenly distributed fashion. Inclusive of canopy thermal losses but exclusive of engine losses, the annual output from a 1,000 m² canopy is estimated as 104 MWhr (horizontal) and 131 MWhr (sloping). Engine losses are expected to reduce these estimates by 25-30%, as described in Section 5 of the paper.

Passive solar heat collection is relatively cheap and easy, so financial metrics (cost per peak Watt, Levelised Energy Cost) for the canopy/engine system are expected to be favorable, especially for the sloping canopy. These metrics are presented in Section 6.

Comment: This peer-reviewed conference paper extends the analysis of Reference [7] to the case where the transparent insulated canopy slopes at the latitude angle. The paper didn’t actually have an abstract, so the text above is an abridged version of the Introduction.

[10] N.G. Barton, “ECET Boost to Solar-Hybrid Gas Turbines”, in Proc. Australian Solar Energy Society Conference, Sydney, 30 November – 2 December 2011.

In the SOLGATE project, a solar-hybrid gas turbine system was developed, built and tested (ORMAT et al., 2005). The concept involves gas turbine systems in which air is first compressed, then heated through a combination of concentrated solar radiation and gas combustion, and finally passed through a turbine to generate power. The hybrid nature of the system enables savings on fuel and emissions, but also power delivery when the sun is not shining. The exhaust from the gas turbines is very hot, and therefore suitable for some kind of bottoming cycle to boost the output.

Recently, the author (Barton, 2012) analysed a new bottoming cycle based on evaporative cooling of hot air at reduced pressure. This concept is called the Expansion-Cycle Evaporation Turbine (ECET). Barton showed that the ECET can typically boost the output of utility-scale open-cycle gas turbines by around 20%, without any extra fuel consumption or emissions, and at a specific capital cost expected to be no more than that for the upstream gas turbine. With these attributes, the ECET is expected to be suitable for peak duty in the electricity grid.

The present work investigates the suitability of the ECET to boost the power output of solar-hybrid gas turbines such as in the SOLGATE project. Power boosts of approximately 20% are expected. The Levelised Electricity Cost (LEC) with ECET boost is also compared to that for two other bottoming cycles – the Rankine steam cycle (which gives the solar-hybrid combined-cycle gas turbine) and an Air Heat Recovery Turbine Unit (AHRTU, Romanov et al., 2010). In these comparisons, the three bottoming cycles give a 10-14% reduction in the LEC when compared to the original solar-hybrid gas turbine. The LEC for the ECET-boosted system is marginally lower than that for the Rankine-boosted system, which in turn is marginally lower than that for the AHRTU-boosted system. The percentage by which the LEC is improved by the bottoming cycles does not depend strongly on the capacity factor.

Comment: This peer-reviewed conference paper compares various bottoming cycles for a solar-hybrid gas turbine. The ECET (see reference [8]) is one of the candidate technologies.

23 January 2012