‘Where have you been?’

here. there. and in between.

currently in Bangalore, India.

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one engine design

here’s one design of the engine

absorber

 

selective absorber and heat pipe structure

 

engine ‘head’

 

quencher engine head

 

 

 

swings with the sun

 

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heat pipes – very efficient heat transfer devices

http://www.cheresources.com/htpipes.shtml describes ‘heat pipes’ well

‘A heat pipe is a simple device that can quickly transfer heat from one point to another. They are often referred to as  “superconductors” of heat as they possess an extra ordinary heat transfer capacity and rate with almost no heat loss’

‘a heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. The wick provides the capillary driving force to return the condensate to the evaporator’

heat pipe moves heat from one end to the other

here’s another definition of the heat pipe:

‘A heat pipe is a heat transfer mechanism that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces’

‘Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid’ (and water works well at the 80-90-100 deg C temperatures we’re interested in working at) ‘in contact with a thermally conductive solid surface turns into a vapour by absorbing the heat of that surface. The vapor condenses back into a liquid at the cold interface, releasing the latent heat. The liquid then returns to the hot interface through either capillary action or gravity action where it evaporates once more and repeats the cycle’

some useful facts about heat pipes:

there are no mechanical moving parts, and no maintenance

the advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat

heat pipes can be made in many different shapes and sizes, including planar or flat sheet like structures

very interesting!

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thermally coupling evaporator and condenser

here is one way to do it

condenser coiled around evaporator – heat captured from condensing steam preheats water

the absorber sits towards the top of the device so that it can look at the sun and within the engine, it sends heat down towards the top of the water in the evaporator. like the heat lamps used in restaurants to keep prepared food warm

water surface heats up and begins to steam. steam is fed into condenser which in this example is tubular and tightly wrapped around the evaporator. as steam condenses and releases heat, it preheats to-be-treated water

maximize heat transfer between condenser coil and water in evaporator

minimize heat loss, for example, by insulating the outsides of both evaporator and condenser

solicitation

the insights of knowledgeable people, perhaps those who are experienced with thermal properties of materials, heat transfer, boiler design, etc are requested to share their expertise, and their inputs are very appreciated

ideas anyone, on suitable evaporator-condenser designs?

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quencher engine’s functional units

quencher engine’s  four functional units are

energy absorber
water evaporator
steam condenser
sludge or waste remover

let’s develop some primary engine design guidelines from  what we’ve learned so far:

i) a ‘selective’ absorber is very desirable as it minimizes IR radiative energy loss

ii) absorber and evaporator are thermally coupled (by being in close proximity with each other for example)to maximize absorber-to-water heat transfer

iii) evaporator and condenser are thermally coupled so that heat recovered from condensing steam may be reused, greatly improving system efficiency

other noteworthy points

i) system efficiency may be further increased by using sub 100 deg C process temperatures, as lower operating temperatures usually mean lower heat losses

ii) with an efficient absorber, continual sun tracking may be unnecessary, allowing stationary absorbers with optimized fixed sun facing positions based on location latitude. this  reduces system cost and increase system reliability by eliminating moving parts

pictorially, these design guidelines are:

absorber

 

evaporator (and waste removal)

 

steam condenser

 

stacking them up, here’s a  representation of the engine’s key functions

 

it is clear that the design begins to point towards a tightly integrated engine unit

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more on selective absorbers

Almeco-TiNOX makes a great selective absorber. it absorbs most sunlight – ’95 % of incident light’ and radiates very little in the infra-red – ’4 %’,  so that around ’90% of the solar energy can be used as heat’

TiNOX is a ‘hi-tech’ selective absorber made of multiple layers, with a quartz anti-reflection coating. it is made using computer controlled process equipment including vacuum chambers and Chemical Vapour Deposition equipment

there are other, simpler, selective absorbers. Dr. Md. Golam Mowla Choudhury from the
Department of Physics, University of Rajshahi, Bangladesh, in his succinct paper titled ‘Selective Surface for efficient Solar Thermal Conversion‘ describes several of them

Dr. Choudhury summarizes the desirable properties  of a good selective absorber:

  • high sunlight-emissivity at 0.2 – 2 microns, low emissivity for greater wavelengths, with sharp transition between the two spectral regions
  • stable opto-physical properties over long term operation at elevated temperatures, repeated thermal cycling, air exposure, ultra-violet radiation
  • if it is a coating (and he talks about several coatings), good adherence to substrate
  • coating must be easily applicable
  • and finally, absorber must be economical and affordable

absorbers such as copper oxide, nickel black, black chrome and cobalt oxide are listed. while these earlier known selective absorbers may not perform as well as TiNOX, they, and others of their kind, are worth a good look, because they may be simple, relatively easy to make and cost effective

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heartening find – efficient large capacity solar water purifier

i learned of a commercial solar water purifier with many units in service in the Middle East. with very efficient performance figures

now these are high capacity systems with stationary installs, like a water plant. (while, as we know, we look to build a portable low capacity very low cost unit). MAGE unit water purification capacities span 1 cubic meter per day (1000 litres) to 50 (50,000 litres)

here’s a 930 litre per day install in Oman, by the sea, operates 24 hours a day with stored sunlight energy

1000 litre per day capacity MAGE solar water purifier

i draw  information from Dr.-Ing. Hendrik Müller-Holst’s presentation ‘Solar thermal desalination for decentralized production of pure drinking water – A technological overview MAGE Water Management GmbH, Presenter: Rudi Gleich, Almeco-TiNOX GmbH, at a US conference july 2010. this presentation includes a very good review of at least 6 solar water purification technologies

first piece of good news – MAGE’s solar purifier uses a selective absorber of the kind we talked about, with high sun-light emissivity and a low infrared-emissivity, made by Almeco-TiNOX. it absorbs most sunlight – ’95 % of incident light’ and radiates very little in the infra-red – ‘4 %’,  (as opposed to almost 50 % for a non selective black surface absorber), so that around ‘90% of the solar energy can be used as heat’. 90% sunlight utilization! in a commercially available selective absorber material. good!

it desalinates water ‘by multiple evaporation of salty waters and
consecutive condensation of the generated humidity‘ the author likens the product’s working method to the natural sun-fired-rain making process

it  ‘recovers energy by a sophisticated arrangement of the condensation-
evaporation unit

recall, it ideally takes 5 kWh of energy to boil 7 litres of 25 deg C water into steam. and the bulk of the energy went into transitioning from 100 degree boiling water to steam (2260 KJ for 1 litre), more than 5 times the energy taken to get from 25 deg C to 100 deg C water (315 KJ for 1 litre). when the steam condenses back to water, it returns the 2260 KJ. as we’d talked, if this heat can be captured and reused, we could process significantly more than 7 litres of water with the 5 kWh we’d captured from the sun (assuming a moderate latitude location, a bright day, and a 1 square meter surface area ideal absorber pointed straight at the sun)

how much more water can we process with this energy? depends on how much of it we capture and  reuse. from their numbers, MAGE’s done a great job capturing and reusing this heat. hence the ‘sophisticated arrangement of the condensation-evaporation unit’ – thermally intertwined to capture as much of the heat as possible – a design idea we’ve talked about with respect to the quencher’s ‘engine’

it ‘requires a process energy of 80°C (175 °F)

not 100 deg C!

i boiled some water in a vessel on the stove and measured its surface temperature using an accurate infra-red non contact thermometer. at a surface temperature of 80 deg C, much of the surface of the water was gently turbulent with masses of small quietly exploding bubbles giving up their tiny pockets of steam. water was turning into steam at 80 deg C, a little of it at a time. hence the ‘by multiple evaporation of salty waters and
consecutive condensation of the generated humidity’. i e evaporating water at the lower temperature of 80 deg C, a little bit at a time, repeatedly. it will still take 2575 KJ of energy to convert 1 liter of 25 deg C water into steam, but they’ve pulled it off at 80 deg C

additional energy is saved by not having to heat the water all the way up to 100 deg C, and instead, topping off at 80. how much energy saved for steaming 1 litre of water at 80 deg C rather than 100 deg C? recall it takes 4.2 KJ for a 1 deg C increase in temperature in 1 litre of water. so 84 KJ saved with the 20 degree reduction. a small amount, compared with the 2260 KJ of the heat available for recapture as steam condenses back to 1 litre of water. but every little bit adds to the efficiency of the unit

as the MAGE units’ numbers tell
unit produces 25 to 35 litres of water per square meter of selective absorber collector area – a very impressive number

recall we’d calculated 1 sq meter suntracking collector collects 5 KWh per day with which we could brute-force-100-deg-C evaporate at most 7 litres of water, without reusing any of the energy released when steam turned back to water

they are purifying 25-35 litres per day – up to 5 times more than we’d first calculated, for the same energy! great news!  and it does not seem like their absorbers even track the sun!

reuse of energy shed by steam when it condenses back to water is essential

other features MAGE points out are that the quality of the purified water is excellent, unit is designed for 25 year life time, unit requires minimal maintenance, water does not require chemical treatment prior to processing, and that product has passed European quality tests

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