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Blog for 2010

 

Sunoba

Renewable Energy Systems

 

 

 

 

 

 

 

 

 

 

25 August:  I’ll be presenting a seminar on the Wellington simulations at the Queensland University of Technology in Brisbane next Tuesday.  Details can be downloaded from https://wiki.qut.edu.au/x/XiJEBQ.  Afterwards, I intend to put a PDF copy of the seminar on this page.

 

 

 

19 August (revised 29 August):  The conference paper I mentioned in my last post had a strict page limitation, so various aspects of the simulations had to be omitted.  I’ve now written a Technical Report to provide these additional details.  I’ve also made a preliminary estimate for cost per peak Watt.

 

# Note added 29 August:

 

A few words on the methodology for financial estimates might be useful.  The output of the combined canopy-engine system was considered in detail over many months, leading to estimates mentioned in the post for 9 August.  These estimates aren’t infallible, but at least a lot of work went into them.  On the other hand, my current estimates for financial metrics are quick and dirty.

 

Based on the 9 August estimates, suppose we have a 1,000 m^2 canopy-engine system that would produce 65 kW peak and 74 MWhr/yr, all losses considered.  Suppose the cost of land, earthworks, frame and canopy is AUD 25/m^2 and the cost of the engine and balance of plant (including any water treatment process) is AUD 1,000/kW.  Then the capital cost is AUD 90,000 and the cost per peak Watt is AUD 90,000/65,000 W = AUD 1.38/Wp.

 

Another common financial metric is the LEC (Levelised Energy Cost).  This requires additional assumptions on interest rates, payback period, cost of maintenance, and cost of water.  Further, the LEC must take into account the debt/equity ratio and the tax deductibility of depreciation, interest and operational costs.  At present, my estimate for the LEC is too preliminary to give here.  I’m working on a spreadsheet that will capture the factors mentioned above (and any others that I discover to be necessary). 

 

Reference: Technical Report 2010-4.

 

 

9 August:  Over the last two weeks, I re-worked the Wellington simulations to account for the dependence of transmission and reflection coefficients on the angle of incidence.  (I used the Mitalas & Stephenson correction, as mentioned in the post for 22 July.)  I’ve also submitted the paper to the peer reviewed section of the 2010 Australian Solar Energy Society (AuSES) Conference.

 

Yes, the correction reduced the power output that I mentioned in my last post.  The latest estimates are as follows (from the abstract of the AuSES paper):

 

“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%.”

 

The AuSES paper was restricted to 10 pages and many details of the simulations had to be suppressed.  I’ll now prepare a Technical Report to document the work thoroughly.

 

Reference: Scientific article [6].

 

 

22 July:  I’ve now completed a full year’s simulations for the Wellington data.  For most of the year, the simulations were based on four days per month at 30-minute intervals.  A few months at the end of the simulations were restricted to two days per month, since by that stage I was confident that there was a strong relationship between power output (in kWhr/(1000m^2.day)) and insolation received (in MJ/(m^2.day)).  As anticipated at the half-way point (see post for 12 July), the annual power output from the 1,000 m^2 canopy was estimated at 112 MWhr (again noting that this estimate does not include engine losses).

 

But I’m not completely happy with these simulations, which I think are too good to be true.  Specifically, I think the transmission of sunlight through the canopy should include a dependence on the angle of incidence, which I have so far not included.  At oblique angles, the amount of sunlight reflected is a greater fraction of the inbound radiation than is the case for nearly normal incidence.  I suspect a sinusoidal dependence on the sun angle, which I need to confirm.  (# Note added 25 July:  I’ve found some useful data and the effect is NOT sinusoidal.  An old technical note from Mitalas & Stephenson, 1962, refers to original work by Fresnel and gives good data.)  The overall effect will be to reduce the annual power output.

 

My deadline for the conference publication is ominously close, but I hope to be able to tweak the model to include the features mentioned above and to meet the deadline of 8 August.  I’d better get started!

 

 

 

12 July:  Today, I completed the first half of the Wellington simulations.  General features of these simulations have been described in previous posts.  My evaporation engine has now been simulated for the months of March-June 2009 and January-February 2010, with energy input coming from passive solar heat collection under a 1,000 m^2 transparent insulated canopy.  The simulations are for 30-minute intervals for four days per month.  The following two conclusions are noteworthy:

 

·         The results show a clear relationship between power output (in kWhr/(1000m^2.day)) and insolation received (in MJ/(m^2.day)).  This enables prediction of the overall performance over a full year.

·         Based on simulations so far, the annual power output from the 1,000 m^2 canopy would be about 112 MWhr.  (Once again, please note that the simulations include canopy losses but not engine losses.)

Reference:  This work will be presented at the 2010 Conference of the Australian Solar Energy Society.  My deadline to complete simulations for the remaining six months and write up the paper is 8 August 2010.

 

 

 

22 June:  The Wellington simulations continue … 

 

As described in earlier posts, I’m simulating the performance of the evaporation engine over a full year at a suitable inland location, namely the city of Wellington in inland New South Wales.  The engine is powered by passive solar heat collection under a 1,000 m^2 transparent canopy with insulation and low-emissivity coating.  Weather and insolation data has been obtained from the Bureau of Meteorology.  At regular intervals during the day, I calculate the flow-rate through the system (comprised of canopy plus engine) to optimise the power output.  This takes time, and a huge number of mouse-clicks, and my original goal of 15-minute intervals has been replaced by 30-minute intervals.  Also, I’m not going to simulate every day; that would be just too much.  Instead, I’m carrying out simulations on the basis of one day per week.  That should give a representative sample.

 

Full results will be presented at a conference later this year.  For the moment, let me mention the best daily output so far.  This was 22 January 2010, a lovely day for solar energy.  The maximum temperature was 38.6°C, with 12.5 hours of sunshine and 32.94 MJ/m^2 insolation on a horizontal surface.  As an advantage for my engine, the relative humidity was low.  Shown below at 30-minute intervals are the inlet temperature (given by a spline approximation to the data) and the optimal power output.  The maximum power output predicted was 89 kW and the total output for the day was 657 kWhr.  The temperature under the canopy peaked at 122°C, and typical canopy losses amounted to about  40% of the inbound radiation in the middle of the day and 50% in the morning and late afternoon.

 

A reminder – these results include canopy losses but not engine losses, which are a topic of ongoing investigation.

 

 

Reference:  This is work in progress.  Results will be presented at the annual conference of the Australian Solar Energy Society in December 2010.

 

 

 

11 June:  I have now revised my canopy heating model and applied it to simulate the daily performance of the BEE.  The model now incorporates transmission, reflection and absorption coefficients that are based on published data.  These apply for both the visible and infrared spectrum.  Also included in the model are molecular heat diffusion (both through the glass cover and a diffusion zone immediately below it) and convective heat transfer from the top of the glass cover.  The simulations are based on the actual angle of the sun above the horizon and ambient weather conditions, with the latter provided by the Bureau of Meteorology.

 

The figures below show results from 1 March 2009 at Wellington, 305 m above sea level in inland New South Wales.  This, the first day of autumn, was a fine and sunny day with 968 W/m^2 direct irradiance averaged over the day and a total of 25.2 MJ/m^2 insolation on a horizontal surface.  The humidity was also low, with the average of the 0900 and 1500 vapour pressures being 732 Pa.  At each quarter-hour interval throughout the day, the flow-rate under the canopy was calculated to optimise the power output of the evaporation engine.

 

Other features of the simulation:

 

·         The canopy area is 1,000 m^2.  The glass cover is 3 mm thick, below which is a 25 mm transparent convection suppression zone.  Of the inbound solar radiation, 12% is reflected, 18% absorbed in the glass cover and 70% transmitted.  The low-emissivity coating reflects 85% of infrared radiation emitted by the ground under the glass.  The glass cover emits infrared radiation both up and down, and both are included in the heat transfer model.

·         The BEE expansion ratio is 1.9.

·         The output in the period from 0815 until 1800 hours is 441 kWhr.

·         4.4 m^3 of water is evaporated to produce this power.   (Comment: note that desalination of seawater by reverse osmosis requires approximately 4 kWhr per m^3; less energy than this is required to treat waste or brackish water.  Also the average annual rainfall in Wellington is 650 mm, so that the average rainfall run-off collected per 1,000 m^2 is approximately 1.8 m^3/day.)

·         These results include canopy losses but not engine losses, which are the subject of ongoing investigation.

 

 

 

 

 

Reference: the methodology and full results will be presented at the 2010 annual conference of the Australian Solar Energy Society.

 

 

 

11 May:  I’ve finally discovered on the internet some experimental data for transmission, absorption and reflection coefficients for glass, either clear or with a low-emissivity coating.   These coefficients are shown as a function of wavelength, so the results are applicable to both the sun’s spectrum and infrared emissions from heated surfaces.  This will enable me to refine the heating models described in the last two posts.  That’s work in progress; results soon.

 

 

21 April:  I have now simulated the optimal performance of the BEE based on passive solar heating.  The key point is the air flow-rate.  Increasing the flow-rate decreases the temperature of the air, and so reduces the work available per cycle.  On the other hand, increased flow-rate means less canopy losses and greater mass flow of air to the BEE.  Where is the optimum?

 

These simulations involve a 1,000 m^2 canopy made of transparent material like ‘bubble wrap’, 20 mm thick with thermal conductivity equal to that of air.  It is coated with a low-emissivity coating, ε = 0.2.  The ambient air is 25°C at relative humidity 53%.  The temperature of the injected water is 20°C.  The expansion ratio of the BEE is r = 1.8 or 2.0.  The solar radiation is Σ = 350, 500, 650 or 800 W/m^2.

 

The figure below shows the power output as a function of flow-rate for the various values of r and Σ.  At optimal flow-rate, the temperature at the outlet of the canopy is less than 137°C for all the insolation values.  The simulations include canopy losses but not thermal or mechanical losses in the BEE.  At optimal flow-rate for the highest insolation (800 W/m^2), the overall system efficiency is just under 10% and the power output is 78 kW/1,000 m^2.  Actual losses in the BEE will reduce these estimates; by how much is a matter of ongoing investigation.  So, you see the philosophy of the BEE in a nutshell – gather the heat cheaply and passively, and then convert that heat energy directly to power without any heat exchangers.  It is all a matter of how well the job can be done, and at what cost.  And that’s why I need investors to help me build a prototype.

 

 

Reference: Technical Report 2010-3.

 

 

 

29 March:  Over the past two weeks, I’ve been looking again at heating of air under a transparent insulated canopy.  My original motivation for invention of the BEE in 2004 was based on the desire to convert heat energy in hot dry air directly into power.  The energy conversion was to be achieved without heat exchangers, and the hot dry air was to be obtained by passive solar heat collection.  About five years ago, I developed a simulation model for solar heating that included airflow, incident solar radiation, and insulation of the canopy, but did not include the effect of radiation losses.   I have now included the radiation losses.  I think it should be technically possible to design and build a canopy that will produce air temperatures up to 140°C.  Under ideal conditions (strong insolation, good insulation, low-emissivity coating on the canopy), theoretical power outputs of up to 75 kW should be achievable from a canopy of area 1,000 m^2.   After allowing for inevitable losses, the actual power output from a canopy of area 1,000 m^2 might be up to 40 kW.  These matters have been written up in Technical Report 2010-2.

 

 

 

15 March:  I’ve now finished Technical Report 2010-1 on the effects of incomplete evaporation during re-compression.  This involved several weeks of computer simulations.  In brief, the work reveals the trade-off between energy gained as an excess of water injected versus the extra energy required to purify and inject the water.  I’m heartened by the results, which indicate that the BEE can be run a bit faster than I expected.

In other things:

(1) Work on the Business Plan has been suspended at a reasonably advanced stage of preparation.  To make further progress, I need input from potential investors and collaborators.  It’s a current priority to identify these people. 

(2) I still have some theoretical results from last year that need to be properly documented – another Tech Report is required.

 

 

 

20 January:  Thanks to all you folks who keep contact with this web site.  Based on the web stats for the month so far, January might produce a record number of hits on the site.  Yes, I’m still working on my inventions, both in a technical sense and commercialisation.  As mentioned in the page for investors, the plan remains to modify the existing experimental engine to test new concepts, and then to proceed to a full-sized prototype.  Details will depend on preferences of industrial collaborators, still to be identified.  I’m currently working on drafts for three Technical Reports and the Business Plan.  In February, I’ll be presenting some results to my mathematical colleagues in New Zealand and then recharging my batteries with a short holiday.

 

 

 

© Sunoba Pty Ltd

29 August 2010