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Science communication is important in today's technologically advanced society. A good part of the adult community is not science saavy and lacks the background to make sense of rapidly changing technology. My blog attempts to help by publishing articles of general interest in an easy to read and understand format without using mathematics. I also give free lectures in community events - you can arrange these by writing to me.

Tuesday, 27 September 2016

Parallax - Measuring Distances to Stars; Visual Depth Perception, View from Moving Trains; Locating Virtual Images

Parallax is the apparent change in position of an object relative to distant background objects resulting from a change in position of the observer.
Parallax means change and is derived from the Greek word parallaxis.

When traveling in a fast moving train, children often ask questions about the rotating landscape ; the video link demonstrates very well this example of parallax.  

Schematically, the slide (adapted from Wiki) explains how parallax works: 

You can observe parallax if you first view the thumb of your stretched out hand against the objects near the wall with one eye closed. Now viewing the thumb with the other eye will show a shift in its previous position against the background objects. Do try this simple demonstration yourself
The two eyes see the thumb in different positions - actually our brain corrects for this change in apparent position as seen by the two eyes to provide one sharp image of the thumb. From this information (parallax), the brain also estimates the distance of the object from us - kind of 3-D perception already built in the processing of images we see with our eyes.  Many birds and insects, do not have much overlap in the fields of view for the two eyes and tend to move their head sideways or up and down to generate depth perception.  (This is called motion-parallax).

We now understand what parallax is - and that it is already useful for depth perception for humans.  What else is it good for?  As it turns out parallax is of fundamental importance in many fields.  I shall look at two in detail here - measuring distances to stars and locating virtual images.

Distances to Stars:  The first step in understanding the Universe is to measure its size - how far stars and galaxies are from the Earth.  Such distances are measured through a series of methods that have overlapping validity - a cosmic distance ladder.  Parallax method is the base rung of the ladder and other methods are calibrated using parallax measured distances as standard.
The distances involved are very large and require a large base line - biggest possible separation between the two points of observation.  The change in viewing direction - parallax angles - also get smaller as the distance to a star increases.  The biggest distance available to us is the diameter of Earth's orbit around the Sun - roughly 300 million kilometers.  The average value of the radius of Earth's orbit is very nearly 150 million km and is called the Astronomical Unit (AU). The following slides explain how distances to stars are determined by the parallax method:

The parallax method defines Parallax Second or parsec or pc as a new unit of distance for astronomy. 

 "A star with a parallax of 1 arcsec has a distance of 1 Parsec (pc). 

One parsec (pc) = 206,265 AU = 3.086 x1013 km.

Another unit of distance that is frequently used in astronomy is a Light Year (Ly).  Light travels 300 million km in one second.  

The distance traveled by light in a year is one light year. 

A light year (Ly) = 0.31 pc = 61,270 AU = 0.96 x 1013 km.  

For completeness:  1 pc = 3.26 Ly

From the slide, we notice that the distance D in pc to a star is equal to 1AU divided by the parallax p in arcsec.

The parallax angles are generally very small and difficult to measure.  Temperature fluctuations in the atmosphere blur the star view and limit measurements to 0.01 arcsec or 100 pc - no more than 100 stars are in that range. 
Space based telescopes Hipparcos (launched by ESA in 1989) provide a better more accurate measurement of star parallaxes down to 0.001 arcsec or distances to 1000 pc. Hipparcos has measured distances of about 100,000 stars. 1000 pc is still only a small fraction of the Milky way that is 30,000 pc across.
ESA launched Gaia in December 2013 to chart a 3-D map of the Milky Way to reveal the composition, formation and evolution of the Galaxy.  Gaia is designed to measure positions of more than 200 million stars to an accuracy of 0.00001 arcsec (distances to 100,000 pc).
On 13 September 2016, Gaia has published the precise position on the sky and brightness of 1142 million stars.

Locating Virtual Images:  In a somewhat less esoteric application, parallax may be used to locate virtual images formed by mirrors and concave lenses.  Consider the case of a plane mirror - you see your image formed but where is it? How do you pin it down?  We can do it by using parallax.
The slide shows how a virtual image is formed in a plane mirror:  But where is it?
In the formation of the virtual image, rays of light from each point do not penetrate the mirror.  They are reflected back to you and appear to come from a point behind the mirror.  Geometrically, using the laws of reflection, it can be shown that the image is exactly the same distance behind the mirror as you are in front of it.  But can we locate it?

We can locate the virtual image using parallax method.  the idea is as follows:

If you view the two pins in the following slide along the line of sight shown then they will appear to overlap.  But on moving the head sideways, they separate in a particular manner that helps you to decide which of the two pins is nearer to you.  

Well,this is essentially what you need to do.  Place a pin at the back of a plane mirror and change its position until the parallax between the pin and the virtual image is removed - they move together as one unit as you move your head sideways.  The position of the pin is the position of the virtual image.
Convex mirrors and concave lenses also form virtual images. These can be located following the same method that I have describe for a plane mirror.

Wednesday, 21 September 2016

Human DNA Radiation Tolerance is Increased by the Unique Protein in Water Bears (Tardigrade)

In my 2010 lecture on the search for extraterrestrial life, I had discussed how some animals are known to survive extreme conditions of heat, radiation, pressure, vacuum etc. This is important because tolerance of such extreme conditions would indicate that life might have evolved even on planets which are on the periphery of the habitable zones. I reproduce a few slides from my lecture in the following:

It is clear that these species have evolved to withstand some of the most inhospitable environments imaginable. 

Tardigrade or Water Bears or moss piglets are one of the hardiest of animals.  They are tiny (0.05 to 1.2 mm long) and live near water. In adverse conditions of extreme hot or cold, very high pressures, space vacuum or intense UV/X-ray radiation water bears shrink, dehydrate and put metabolic activity on hold.  Dehydrated water bears can survive for years but come back to life when in contact with water which they need to grow and reproduce. 

The question is - how do they manage to survive the extreme conditions?  Such unusual tolerance of tardigrades has long fascinated researchers; however, the molecular mechanisms enabling such exceptional tolerance have remained largely unknown.  Water Bears Video
A study of tardigrade genome, published in Nature Communications this week has provided some amazing insight into the tolerance of tardigrade to extreme conditions.
I am reproducing their conclusions in the following slide.  (Apologies for the somewhat formal language of the slide but I feel it presents the researchers' conclusions in a pristine way)
What I find the most exciting is the demonstration that the unique proteins can also protect human cells against X-rays induced DNA damage and improve human tolerance to radiation.  This is a game-changer and as the authors say, there could be a bountiful source of protection genes and mechanisms.  

An interesting question to ask is how did R. varieornatus tardigrade acquire the genes for the unique protein - Hashimoto et al. call the protein Dsup (damage suppressor). 
This has been a controversial issue with some previous work suggesting that tardigrade acquired many of their genes from bacteria through a process called horizontal gene transfer (HGT) - an important mechanism for the evolution of many organisms.  This study - which is by far the most extensive and is a full genome sequence of a tardigrade - sets an upper limit of 1.2% on the contamination by foreign genes and claim that the protein Dsup is uniquely developed by the tardigrade itself.  
It was also demonstrated that Dsup protein affords DNA protection without impairing cell viability and is suitable for application to confer tolerance to other animal cells
In fact, Dsup-expressing human cultured cells exhibited better tolerance to 4 Gy of X-ray radiation. (One gray or Gy is the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter). 

Friday, 16 September 2016

Will Future Water Crises Destroy Our Civilization? - Probably Not - But Only if We Start Paying Attention Now...

(Click on a slide to see its full page image; press ESC to return to text)

Freshwater is one of the four pillars on which our civilization rests - the others are food, energy and the climate. Humans have adversely impacted all - in a big way - no wonder the new epoch is called the anthropocene. How long will it last? - Humans will have the control on that too!

I had looked at food in a recent blog. Water is fundamental to our survival - water is a marvel of nature with such unique properties that without these the very existence of life would not be possible.  I recommend highly that you look at the Wiki article to feel amazed how this simple molecule can express myriad of such wonderful properties.

To put the subject in context, I shall take a brief historical look before discussing the present situation.  Then, I shall detail a few ideas about tackling the water crisis to ensure adequate freshwater supply to all of the world population in 50 years time. 

Historical Context:  Climate change and water in particular have been linked to the demise of many of the great ancient civilizations.  The first of the following two slides provides a summary and the second slide tells a case history of the Indus Valley Civilization (IVC) to demonstrate that these were advanced societies with well thought out and established system of civil engineering, governance etc.

IVC lasted over 6000 years - much longer than the present western civilization.  I am sure the quality of life was also very reasonable - and they say that people lived peacefully with no wars. 
We note that the populations in these ancient civilizations were relatively small - a few million people - and they always developed in areas where water supply would be plentiful. However, they did not have the means to shift a lot of people over vast distances to escape from natural climate changes and resulting water shortages.

Current Situation: In our technologically advanced societies, we understand the global conditions very well and have good scientific understanding of the Earth's hydrological cycle.   

The amount of water in the world is fixed; freshwater is a mere 2.5% of the total.  To make matters more difficult, accessible freshwater is only 0.008% of all water. (see slide).
Of the 0.008% global freshwater, we use almost 90% in industrial and agricultural activity. Domestic use accounts for a mere 10% of this freshwater.  Growing population with improving living standards will create extra food demand - higher agricultural output will be needed with corresponding call for extra water resources - the current situation looks hopelessly unsustainable.  

To understand the magnitude of the problem facing the world, we need to look at the way water is used in agriculture to produce food. 

Naturally, large populations settled in area that had big promise of rain-fed irrigation - water was plentiful to grow food.  Then, we also learnt that irrigation is helped by drawing water from rivers and aquifers (groundwater or fossil water).  Even today, 80% of the agriculture production is by rain-fed water.  The three main users of freshwater - agriculture, industries and domestics - share the global freshwater supply.   70% of global freshwater is used for irrigation that produces the remaining 20% of the food. This is the biggest call on global freshwater supply and there lies the problem.
Global warming (GW) is an established fact - our Earth is heating - both land and seas. This has many consequences that are well researched and reported.  For our context, the main point is that warming of the seas increases evaporation and also disturbs the exchange of energy between the oceans and the air in the atmosphere (remember that water in the oceans absorbs the greatest part of the solar energy that falls on the Earth and this energy is then redistributed through global air and water circulation patterns).  Air circulation carries moist air to various parts of the globe and causes rainfall - that is responsible for 80% of food production in the world. GW is predicted to affect the way air circulates with consequences in terms of shifting rain fall patterns.  Areas that currently receive large amounts of water may experience droughts; but that is where the populations are and agricultural land produces most of the food.  The result will be loss of agricultural yields, famines and mass migration.
A second threat is our dependence on groundwater for irrigation and domestic use.  Groundwater is water that is trapped underground in aquifers.  Some aquifers are closed and are not replenished by rainwater while others do get topped up by rain.  If the withdrawal rate of water from aquifers is greater than replenish rate then total amount of water available will be reduced and is indicated by the increasing depth that pipes must be dug to reach the water levels. The following slide indicates the regions of current water stress:
The situation is even more serious for aquifers that are closed (not recharged by rain) as in the Middle East.  An interesting case study is of Saudi Arabia who once sat on a large body of water in an ancient aquifer.  Around 1970, Saudi regime decided to become self sufficient in food by using water from their aquifer.  The slide tells what happened... 

Climate change will affect many areas by either making them too hot to live and/or by creating drought conditions.  This will result in mass migration of populations creating many social, political and financial problems.  Resettling of refugees will also require additional strains on the food and water resources.
The above discussion emphasizes the need to be careful how we plan our policies and actions. Future is uncertain and climate change effects must also be taken into account in forming policies. Some common sense steps pop up:

Currently, agriculture uses 70% of the available freshwater.  If we can reduce the water usage in agriculture by 20% then domestic water availability will more than double from 10% to 24%. This alone would be enough to supply potable water for 10 billion people.  Good agricultural practices like drip irrigation can achieve this but are still not widely used.

Water from aquifers should only be withdrawn at rates that does not reduce the total volumes available - rate of withdrawal must not exceed recharge rate.  This will impact on irrigation but must be taken into account as a prudent step.
Coastal areas have desalinated seawater to provide freshwater - desalination is a energy intensive process - and the use of fossil fuels for desalination contributes to global warming.  Better technology here will help.
Water is a precious commodity and recycling of waste water is a must.  Good progress has been made in the past decades in recycling technologies.
Addressing water scarcity issues:   I float a few ideas in the following - with particular emphasis on water desalination processes:
a. Agriculture uses 70% of freshwater:  How does one reduce the consumption of water used in agriculture?  In my previous blog, I had addressed the question of sustainable food supplies.  Switching our diet from animal protein to plant protein will be enormously effective and will eliminate the need to expand arable land, at the same time reducing the water requirements. I had discussed hydroponic farming that uses water (and land) extremely efficiently.  3-D printing of food holds a big promise in terms of reducing waste in food production.  Reducing loss of food in storage, transportation and in homes can save up to 15%.  

In traditional agriculture, drip-irrigation can save up to 50% water requirements.  Drip irrigation has a serendipitous discovery that is worth recounting here:
In the 1930s, a water engineer was visiting his friend in a desert in Israel when he noticed a line of trees with one member that was much taller and more robust looking than the others. He did a little digging, literally, and found that a household water line running along the tree line had sprung a leak in the area of that one tree and was feeding it with a steady drip drip drip of water.  The wet area spot on the surface didn't seem like much, but down below was a large onion-shaped area of juicy soil.

b. Drinking Water:  While there is much scope of reducing the use of freshwater in agriculture - this can not be the whole solution. Many of the world's coastal areas suffer from lack of drinking water. Sea water contains about 3.5% salt and is not good for human consumption or for agricultural use. Some areas, as in the Middle East, also experience drought conditions regularly. Many urban areas, particularly in developing countries, have poor water supply systems. Currently, more than 1 billion people do not have supply of safe drinking water. The mantra for supplying potable water has to be to minimize waste, collect and recycle all available water.
Rainwater Harvesting has been practiced in semi-arid areas for centuries but has become much more common recently.  Some countries require new homes to have adequate rain collection systems.  Rainwater is essentially pure water and is suitable for drinking.  Rainwater collected in tanks is still quite clean and is suitable for general household purpose and also for irrigation.  Rainwater harvesting is an excellent example of harnessing a resource that is essentially cost free, is widely available and is already helping in mitigating water scarcity in semi-arid areas around the world.

Recycling:  Recycling water makes sense.  An extreme example of recycling water is at the International Space Station (ISS) where 93% of all water is recycled.  Treated wastewater from residential buildings and industries can be reused for many useful tasks such as agriculture and landscape irrigation, industrial processes, toilet flushing and also recharging groundwater aquifers. Recycled water can satisfy most water demands, so long as it is adequately treated to ensure water quality appropriate for the use.

Water Desalination    has long been used in water-scarce regions to increase drinking water availability.  Currently, 11 billion gallons desalinated water is produced globally everyday.  Water desalination (WD) not only holds great promise for solving future water scarcity problems but also it is being used for desalting brackish (moderately saline) water and for softening and removal of organics and impurities in groundwater. Unfortunately, the quality of conventional freshwater sources is being degraded by pollution etc and a process to improve general water quality is required. Membrane separation is a cost-effective way to achieve this.

In the words of John F Kennedy:  If we could ever competitively, at a cheap rate, get freshwater from seawater...(this) would be in the long-term interests of humanity which could really dwarf any other scientific accomplishment.

WD can be performed either by thermal distillation or by membrane separation. The following two slides are adapted from Banat.

Thermal Distillation:  Over 60% of the world's desalted ocean water is produced by boiling seawater to produce water vapour that is then condensed to form freshwater. 

Thermal distillation plants produce water with salt concentrations from 5 to 50 parts per million - seawater salt content is 35000 ppm.  Thermal distillation is highly energy intensive and so far the energy source has been fossil fuels which is damaging for the environment and the climate.  Middle East with about 50% of the global desalination capacity has plenty of cheap oil and there has been little incentive to use renewable energy sources.  In fact, it is the drought affected areas that need to desalinate water and these are the areas where solar energy is plentiful.  With prices of solar cells plummeting, it is expected that solar renewable energy will replace fossil fuel for thermal distillation.  Currently the energy cost is about 13 kWh or one US dollar per cubic meter of thermally desalinated water.   

Membrane Separation Processes (MSP):  

Nano-filtration (NF); Reverse Osmosis (RO)

Membrane separation processes (MSP) use a membrane - a filter or a semi-permeable membrane - to produce drinking water in a more cost-effective manner than thermal distillation.  Water can have impurities that are either suspended (microbes, chemicals, dirt etc.) or dissolved (salt, minerals etc.). These are detailed in the following slide

For desalination where sea salt (NaCl) is dissolved in water at a concentration of 3.5% and is in ionized state, the process used is reverse osmosis (RO).  RO was developed in the 1960s and represents a serious breakthrough in desalination technologies. Let me explain this in more detail:

In osmosis, two solutions with different salt concentrations and separated by a semi-permeable (also called partially or selectively permeable) membrane (SMP) have a tendency to make the concentration equal in both parts.  This is achieved by the solute molecules (water in our case) passing through the SMP from lower concentration side to higher concentration side.  SMP does not allow impurity molecules (salt ions in our case) to pass through - only water molecules can. 

In reverse osmosis a change in the direction that salt water naturally seeks is achieved; and water molecules from high concentration (seawater side) move through the membrane to the low concentration side (pure water). This is done by applying high pressure on the seawater side of the system.
In fact, the RO water is so pure  - de-ionised or de-mineralized - that it is mixed with some original salt water to be suitable for drinking.

RO systems are good to desalinate all types of water and also for the removal of contaminants like radio-nuclides, nitrates, arsenic, pesticides etc.  Nano-filteration (NF) uses lower pressures and is useful for the treatment of hard,  coloured water, viruses, and also high organic content feedwater.  
The cost of RO desalination is about half of the cost of thermal desalination; energy costs are 30% and membrane replacement costs (a typical lifetime of the membrane is 5 years) 10%.  These costs have been coming down - membranes are 5 times cheaper since 1970.  In traditional water-short areas the costs of desalted water are already competitive with conventional water sources.

It is hoped that with renewable energy becoming more widely available with improved affordability, water desalination will not only supply sufficient potable water to water-scarce coastal areas but also we shall be able to use brackish (less salted) water, wastewater etc.  This will increase the available water pool size.  Additionally, recycling of water with membrane technology holds the promise of satisfying domestic residential water requirements.

New technologies are coming up that could make filtration much cheaper and efficient.  See

Final Word:  I have considered the ways in which sufficient potable water can be made available to 10 billion people in about 50 years time.  The technology is there already and in the future, systems will become more efficient with even lower costs. Irrigated agriculture water supply situation requires more widely adopting methods like drip-feeding, hydroponic farming that are well tested but requires better information to be made available to farmers.  Switching from animal protein to plant protein will solve the water problem in a big way but this requires a change of culture and habits - these tend to be difficult asks - but one hopes that with good official backing (UN is already promoting the shift) from individual governments, people may be convinced over the next decades to consume more plant proteins.
But then one also has to ask the question  - that with plentiful food and water available, why in the world today over one billion people do not have enough to eat and sufficient potable water to drink?
My biggest concern remains climate change.  Shifting rainfall patterns can cause havoc for indigenous populations and forced mass migration from such areas can destabilize our already fragile geo-political order. Is it too late to do something substantial to mitigate the effects of climate change? 

Wednesday, 7 September 2016

Hi-Tech Wearable Clothing can Save Energy on Space Heating/Cooling in the Winter/Summer: Personal Thermal Management

Blog Contents - Who am I?

Humans are warm blooded with optimum skin temperature at about 35C.  A person also produces about 100 Watt of power through metabolic activity (click here for an estimate) that must be discarded to the surroundings - via radiation, vaporization and convection.  Relative importance of a loss process is determined by the difference in the temperature of the body and the surrounding air (more on this later).
In winter, when the surrounding temperatures are low, we lose more energy than we produce and feel cold - wearing several layers of clothing traps extra air and reduces energy loss by convection. Wool is very effective. Central heating (CH) is set to maintain temperature inside the building at a comfortable 22C.
In summer, the surrounding temperatures can be much higher. The body does not lose heat efficiently and air conditioning (AC) is optimally set at about 26C.  To increase heat loss by convection and vaporization, the best strategy would be not to wear any clothes - going naked is not always possible - and one resorts to wearing cotton clothes to keep cool. Cotton is also permeable to water vapour and allows efficient vaporization to occur. 
Both CH and AC are energy intensive and form the biggest slice of a family's energy bill.  They also have serious impact on climate change through the emission of greenhouse gases (GHG). We are advised to reduce CH to 18-20C range and use less AC to help save the planet. Wear more layers and chunky woolens to keep warm in the winter.  I had estimated that sensible conservation measures could save £2 billion per year in residential energy costs in the UK alone.  Energy conservation is a must for our societies and I think everybody should play their part in reducing energy consumption.

The research work at Stanford by Professor Yi Cui and colleagues about personal thermal management (PTM) has the aim of managing heat loss from individuals obviating the need to maintain the temperature of the surrounding space at levels currently used.  PTM is achieved by a clever design of the clothing material using nano-technology (NT).  Before I explain how this is done - let us look at the way the outer surface of the human body gets rid of the 100W of internally generated heat.  
I have redrawn the following slide from Hardy and DuBois and it shows the relative importance of the three mechanisms of heat loss from the human body.  Inside buildings the temperatures are generally less than 30C and radiation is the most important mechanism of heat loss. Vaporization is essentially sweating and becomes overwhelmingly important when temperatures reach above 33C - at this point radiation and convection are essentially switched off as they depend on the temperature difference between the skin and the surroundings.
For the interesting temperature range of 20 to 26C, radiation is the most important mechanism and I shall explain this process in more detail.  
The Stanford group's hi-tech fabric also focuses on radiation losses for PTM.

Essentially, all bodies radiate energy (heat and energy are exactly the same and either term will be used in this discussion). How much? - depends on the temperature of the body and the surroundings, and on the surface area of the body. For our discussion, we shall consider the human body to radiate as a blackbody - meaning that its  efficiency of emitting radiation (emissivity) is unit. A mirror has an emissivity of nearly zero.  Human body is a good approximation of a blackbody. Now comes the interesting bit with some physics:

If we look at the blackbody radiation closely - we find that it is composed of a range of wavelengths.  The range depends on the temperature T(in Kelvins) of the emitting surface; the intensity of radiation is a maximum at a wavelength (in microns) given by 2898/T(in K) - this is called the Wien's Law.  Humans at 35C or 308K emit radiation centered around 9.4 microns or 9400 nm covering a range from 7 to 14 microns.  This is in the infra-red (IR) region of the spectrum (see slide below).
The figure also shows blackbody radiation curves for some stars - in fact it is from these types of measurements that we infer the surface temperature of a star. Our Sun has a surface temperature of about 5800K and the peak of emission is in the visible region (2898/5800 = 0.5 micron or 500 nm) with significant radiation in the ultraviolet and infrared (IR) parts of the spectrum.

PTM on a hot day:  Obviously we need to let the radiation from the body to pass through the clothes we are wearing. Cotton and other textiles are quite efficient in absorbing IR radiation from the human body.  They do allow air convection and help to cool the body by convection and vaporization but not by radiation. Stanford group have developed a nanoporous polyethylene (nanoPE) that does not absorb human body IR radiation. NanoPE has interconnected pores that are 100 to 1000 nm in diameter.  This pore size strongly scatter visible light (400 to 700 nm) and makes nanoPE opaque to human eye - appears white like normal textile fabric.  NanoPE is also highly transparent to IR wavelength and interconnected pores enable air permeability and can be water-wicking (to help vaporization) when the PE surface is chemically modified to be hydrophilic.  The following slides provide a summary:

NanoPE is commercially available and costs ~$2 per square meter - comparable to normal textile prices. The following slide describes how the nanoPE material is modified to be a commercially useful fabric with cooling properties superior to cotton textile.  In the words of the Stanford group...

PTM on a cold day:  NanoPE looks great on a hot day as it allows us to lose heat efficiently.  What about winters when we wish to retain as much of the body heat as possible?  For this, we just need to switch to silver nano-wire (AgNW) embedded fabrics.  (See slide below). The metallic nano-wires form a conductive network that is highly insulating because it reflects human IR radiation.  The breathability and durability of the original cloth is not affected because of the nano-wires' porous structure.  Moreover, nano-wires are conductive and additional heating may be provided by using a battery (Joule heating).  
Let me explain further: In winter, the wall temperatures even in a properly heated house can be as low as 20C.  This increases heat loss by radiation - heat loss increases rapidly as temperature difference between human body and the walls gets larger.  Using AgNW fabric, one can keep the room temperature significantly lower - say 16C and still not feel cold.  Currently,  42% of energy used is spent on residential temperature management.   
AgNW fabric works because of the low emissivity (equal to 0.02) of silver and it reflects back over 90% of the IR radiation from the body to be compared to 20% reflected by normal clothing (emissivity equal to 0.8). The spacing between nano-wires is on average 300 nm (1 micron = 1000 nm) while the human radiation has a wavelength of 9.4 micron or 9400 nm.  The radiation essentially interacts with the silver nano-wire mesh and is reflected as from a continuous silver film.
The 300 nm wire spacing is large compared to the size of a water molecule (0.2 nm) and AgNW shows good breathability.
Is AgNW expensive? - Not really.  A coating of silver nano-wires uses very little silver - about 0.1g per square meter. 
The Standford team also found that repeated wash cycles do not affect the durability of the fabric.

While the AgNW fabric is effective for personal wear, it does not replace the central heating systems as the authors might have argued in their report.  A building must be heated to provide protection to water pipes, furniture and many other household items.  Even for personal use, it is not possible to completely cover yourself with clothing and a reasonable ambient temperature of 16 to 18C is desirable.  This is still effective in reducing energy requirements for space heating.
Where this type of clothing may be most useful is for personal wear outdoors when one can cast away heavier bulky clothes in preference to AgNW wear.

Both nanoPE and AgNW have been demonstrated to work in the laboratory.  One hopes that they will work equally well in general use.  This remains to be seen.  

Final Word:   The work on PTM by the Stanford group is a good example of the way new technologies are impacting our day to day lives.  Historically, humans have used clothes made from available materials without serious understanding of how they help in maintaining body temperature during different seasons - cotton was better in summer and wool in winter.   I have tried to demonstrate in this blog how science has provided a proper understanding of mechanisms by which body manages heat loss and in the way new technologies use this understanding to develop fabric and procedures that make the clothes we might be wearing in future just a little less bulky and more comfortable.  The energy savings could be significant and will contribute to a reduction of the environmental damage that humans have unwittingly been guilty of.  Every little help is to be welcomed in this endeavour.

I would like to acknowledge with thanks comments by Professor Yi Cui, Stanford University on this blog.