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Passive Solar Thermal Power Generation

... a Technical Overview

 

Sunoba

Renewable Energy Systems
 

 

 

 

 

 

 

 

  

  

Description: canopy.jpg

 

This is a new concept for solar thermal power generation, initially targeted at an intermediate scale between rooftop and utility-scale production.  Air is heated passively by the sun under a transparent insulated canopy and then fed directly to the inlet valves of a large, slow-revving, multi-cylinder engine.  A typical canopy is illustrated above without the engine.

 

The engine executes a thermodynamic cycle involving evaporative cooling of hot air at reduced pressure.  The speed of the engine is controlled by DC electricity generation from a rotating crankshaft.  The electricity output is conditioned to AC power at the grid voltage.

 

Where would these systems be used?

 

The evaporation engine is suitable for a dry sunny climate.  A significant fraction of Australia would be suitable, as would be the Mediterranean and South-West USA, to name two possibilities among many others.

 

How much power would it produce?

 

 

The loss-free theoretical work output (J/kg air) of the piston-in-cylinder cycle is shown above when the ambient air is at 30°C with partial pressures 99.3 kPa (air) and 2 kPa (vapour).  The results are for ambient air heated at constant pressure to 100, 150, 200, 250, 300, 350 or 400°C before the engine inlet.  The injected water has temperature 20°C.  The work output depends on the temperature and humidity of the air and the expansion ratio of the engine. 

 

For the passive canopy-engine system, the air flow-rate through the engine is chosen to optimise power generation after inevitable thermal losses from the canopy.  Depending on the conditions and time of day, optimality usually corresponds to engine inlet temperatures in the range 90-140°C.  Under excellent conditions, the overall canopy thermal efficiency can be as high as 60%.

 

The figure above displays simulated daily output (kWhr) from a 1,000 m^2 canopy-engine system at Wellington, NSW from March 2009 to February 2010*.  Results are shown for both horizontal and sloping canopies.  Wellington is an inland city at 33°S latitude and 305 m elevation.  The results include canopy losses but not engine losses, which will reduce the power output by 25-30%.  The sloping canopy gives about 25% greater annual output, more evenly throughout the year.

Acknowledgement: weather data purchased from the Bureau of Meteorology.

* N.G. Barton, “Annual Output of a New Solar Heat Engine”, Proc AuSES Conference, Canberra (2010)

   N.G. Barton, “Output of the Evaporation Engine (Sloping Canopy), Proc 2011 Solar World Congress, Kassel (2011).

 

 

Results from the 1,000 m^2 Wellington simulations are shown above for 15 December 2009, a typical summer’s day.  At left is the outlet temperature (°C).  At right is the power output (W, engine losses excluded, as optimised over half-hour periods).

 

After taking all losses into account, the estimated annual output from the Wellington simulations is

 

 

units

horizontal

sloping

peak power

W/m^2

65

60

annual output

kWhr/(m^2.yr)

74

94

 

What is the scientific and engineering basis for these claims?

 

This concept has been investigated since 2004.  An analysis of the thermodynamic cycle of the engine has been published in the peer-reviewed scientific literature*.  Other aspects of the work have been described in five Conference papers and more than a dozen in-house Technical Reports.  The thermodynamic cycle was confirmed in an experimental engine that was built and tested in 2008.  Key aspects of the evaporation engine are protected by patent applications.

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

 

What are the financial metrics (cost per peak Watt, Levelised Electricity Cost)?

 

The Wellington simulations were based on a transparent insulated canopy, 100 m long, 10 m wide and 2 m high as shown earlier.  The following assumptions are made for costs:

  • canopy (land, frame, glass, construction): AUD 25/m^2
  • engine and balance of plant including water treatment: AUD 1,000 per installed kW
  • operations and maintenance expenses: 4% of capital cost
  • interest rate: 8%
  • payback period: 25 years

 

Under these assumptions, the financial metrics are as follows:

 

 

units

horizontal

sloping

Cost per peak Watt

AUD/W_p

1.38

1.42

Levelised Electricity Cost

AUD/MWhr

173

128

 

The general basis for the estimates is that the components are mass-produced and that the 1,000 m^2 canopy-engine system is operated as part of a large installation in Wellington, NSW.

 

The financial metrics are sensitive to both construction costs and canopy-engine performance.  The estimates used will become more robust with further development of this concept.

 

See www.sunoba.blogspot.com for further discussion on the cost of solar power, as estimated for utility-scale installations around the world (both solar thermal and PV).

 

How does this concept compare with CSP and PV?

 

The whole philosophy of this new evaporation power system is based on low-cost power generation from passive solar heat collection. Key aspects that save on costs are:

  • The canopy does not need to be pointed at the sun; heat energy is collected passively.
  • The glass sheets of the canopy will have a low-emissivity coating and convection suppression region on the underneath side.  The canopy will be assembled in situ with the glass sheets mounted on a simple frame.  The canopy is at ambient pressure.
  • The maximum temperatures in the whole installation will be less than 150°C.
  • The engine will be large, slow-revving, multi-cylinder and lightly stressed, with simple mechanical connections to the driveshaft and generator.  Many engine components could be made from plastic or other non-metallic materials. 
  • Air is the heat transfer fluid under the canopy and the working gas of the engine. 
  • The system does not require heat exchangers or condensers. 
  • For the horizontal canopy, the system is economical on land usage.

 

Comparison to Concentrated Solar Power (CSP)

 

CSP systems need accurate mounts and controls for heat collection, especially for two-axis systems.  Collectors for CSP systems are expensive and widely spaced so as to minimise shading losses.  Thermal losses are always present in CSP systems, no matter what collection temperatures are used.  As shown in the following table*, the estimated land use efficiency of the new system is comparable to CSP technologies, either one-axis or two-axis.

 

collector and power technology

η1 (%)

f (%)

η2 (%)

parabolic trough, steam cycle

13-16

25-35

3.3-5.6

linear Fresnel array, steam cycle

8-12

60-80

4.8-9.6

heliostat array, steam cycle

12-16

20-25

2.4-4.0

dishes, steam cycle

27

15-19

4.1-5.1

horizontal canopy, as described here

2-6

90-95

1.8-5.7

 

* Data from www.desertec-uk.org.uk and www.wizardpower.com.au.  Terms defined by:

η1 = solar-electric efficiency = {power generated}/{irradiance on aperture}

 f  = land use factor = {aperture area of collectors}/{total land area required}

η2 = land use efficiency = {solar-electric efficiency} × {land use factor}

 

Comparison to Solar PV

 

As widely deployed at present for roof-top installations, solar PV has efficiencies of 15-20%.  Cosine losses need to be considered in the case when the panels are not actively pointed towards the sun.  The output is low-voltage DC that is converted to grid-voltage AC in an inverter.  Solar PV requires sophisticated manufacturing technologies. 

 

What is the mechanical principle of the evaporation engine?

 

The details of the engine mechanism are currently a commercial secret.  Details can be provided to potential investors under Non-Disclosure Agreement.

 

What about the water consumption in this concept?

 

The figures quoted above include reverse osmosis water treatment as part of the balance of plant.  The feedstock for RO treatment could be saline groundwater, seawater or waste water, e.g. from sewage treatment plants.  In some circumstances, rainfall run-off from the canopy can meet a substantial fraction of the water demand (58% for example in the case of the Wellington simulations of a horizontal canopy).

 

Is it envisaged that energy storage will be used?

 

This concept is well suited to thermal energy storage, for example by passing the hot dry air through a bed of loosely packed pebbles.  This would give “despatchable” power after the sun has set.   Storage would increase the cost per peak Watt but decrease the Levelised Electricity Cost.  Current research on thermal storage will be reported on as soon as possible.

 

How original is this Intellectual Property, and is it encumbered in any way?

 

The thermodynamic cycle for the evaporation engine was originally conceived in 2004 and theoretically analysed from 2004-2007.  Subsequent research has examined various mechanical concepts to execute the cycle, as well as particular aspects such as the time required for injected water droplets to mix with the air and evaporate.  The thermodynamic cycle was tested in an experimental engine that was completed in 2008.  The Wellington simulations were carried out using data either publicly available or purchased from the Bureau of Meteorology.  Key aspects of the work are held as industrial secrets and protected by patent applications.  Patent and internet searches show that the concept is completely original, worldwide.  In summary, the IP outlined here is new and completely controlled by Sunoba Pty Ltd.

 

Where can I find more information?

 

Other pages on this web site will provide general information.  For specific information, please contact the inventor of this technology.

 

 

 

© Sunoba Pty Ltd

22 August 2011