Solar Thermal Energy for the VillageDale Andreatta, Ph.D., P.E.

At ETHOS 2013 we were challenged to start thinking about

village energy, not just cookstoves.• 3 uses for hot water.• 3 ways to use the sun to produce hot water.• All are interchangeable.• One other use for solar thermal (to be

explained last)• Purpose of the talk is to stimulate thinking. • A written report is available.

3 Uses for Hot Water

• Washing water, typically around 45-50° C.• Pasteurized drinking water, 65° C.• Pre-heated water for cooking, 70° C and up.

Bob Metcalf, Microbiologist

Basics of Pasteurization• Pasteurization: A heat process that kills all

disease-causing microbes (pathogens) including cryptosporidium.

• First discovered by Louis Pasteur 150 years ago.• Hepatitis A is the most heat resistant pathogen,

but is killed by a few minutes at 65° C. • Not necessary to boil the water, as many believe.• Heat denatures the proteins, similar to cooking

an egg white (which can also be done at 65° C.

D values (90% reduction times) bacteria in water

seconds at

Bacterium 55°C 60°C 65°C

E.coli 0157:H7 223 67 3

Shigella sonnei 354 54 3

Salmonella tyhpimurium 77 4 <2

Ref: A. Spinks et.al, Water Research 401326-1332 (2006)

Temperatures which kill disease microbespresent in contaminated water

MICROBE KILLED RAPIDLY AT:

Worms, Giardia, Entamoeba,Cryptosporidium 131F (55C)

Escherichia coli, Shigella, cholera, 140F (60C)Typhoid, Rotaviruses, Polioviruses

Hepatitis A virus 149F (65C)

Milk Pasteurization = 160F (71.7C) for 15 seconds

Why is Boiling the Standard Treatment?

• Until 1992 there was no practical way to know whether the pasteurization temperature has been achieved.

• In 1992, the first Water Pasteurization Indicator (WAPI) or “Indicator” was designed.

• Several other indicators have since been developed.

Two Types of IndicatorsPetroleum WaxInitial positionMelts at 65 C

If found here later,65 C was reached

Petrifilm and Colilert Test Results-Raw water (top) and after pasteurization (bottom)

Pre-heated Water for Cooking• If we think of the standard 5-liter Water

Boiling Test, it takes about 1.7 MJ to heat the water to boiling, and typically 15-30 minutes.

• If you pre-heat the water to 70° C, you save 1.05 MJ, and cut the time to boil roughly in half.

• This is 430 g of wood, or 117 g of charcoal, at typical efficiencies.

AquaPak in Use

The AquaPak

• The Aqua Pak was designed as a pasteurizer, made of inexpensive plastic.

• Contains up to 6 liters.• Small glass indicator built into the cap. • Cost is about $2 if made in the developing

world. Currently available, made in San Diego.• The plan is for independent entrepreneurs to

set up a series of factories, copying the product and the process of making it.

AquaPak Test: August 4, 2013, partly cloudy, high temperature 24° C, 5.8 liters

Ways to Use the AquaPak

• Pasteurize 5+ liters.• Warm 2 batches of wash water.• Preheat 2 small batches or 1 large batch of

hot water for cooking.

Other Nice Features

• Built in handle allows it to be carried, with up to 8 at a time with a pole.

• The ultraviolet stabilized plastic lasts up to 4 years.

• Seams typically last 2 years.

SOIL

TROUGHWATER

INSULATION

CLEAR PLASTICDOUBLE PLASTIC LAYER(TOP LAYER BLACK)

AIR GAP

SPACER

The Solar Puddle, A Large Batch Pasteurizer

Basic Solar Puddle, built into the ground.Horizontal dimensions compressed for clarity.

Two benchtop solar puddles: Bubble wrap top layer on left. Two-layer top layer, on right, with top layer removed.

Puddle Results, Calculated, for Various Water Depths

How to Use the Solar Puddle

• For washing, use thick layers of water.• For pasteurization, medium thick layers of

water.• For pre-heating for cooking, use thin layers,

perhaps once in the morning and once in the afternoon.

• Perhaps transfer to an insulated container, or just wrap up the bag.

Pot-based Heaters-Small

Pot-based Heaters-Large

Rocky the Lab Assistant

Rocky’s calling card

Large pot-based heater: 29 liters, July 25, 2013, sunny weather, high temperature 27° C.

Small pot–based heater: 6.8 liters, July 17, 2013,

mostly sunny, high temperature 34° C.

Final Use of Solar Thermal Energy-Sanitation

• Sanitation-dealing with shit-is a huge problem. • Worse in urban areas, but bad enough in rural

areas.• Should be possible to use solar thermal energy

to pasteurize feces.• Couple this with using excreta as fuel or

fertilizer, as well as dealing with feces in a nicer way.