The "efficient airconditioning" debate / Heat stress and cooling

We came across a well intentioned effort to make “efficient airconditioning” (30-80%) more accessible to a larger set of Indians, as a step towards helping adapt to a warming planet and to mitigate some of the footprint and energy demand and cost that will accompany this demand.

Raised a few questions in my head. Trying to understand this better.

To start with, the basics of airconditioning:


Qn : What’s the footprint of all the airconditioning manufacturing, and the delta on the additional energy, even if it’s way more efficient?

Qn : 90% of India saw a reduction in wages post Covid, esp inflation adjusted. This is on a fairly low base. Is this a solution for most of the population anyway?

Qn : Given both the basic physics of airconditioning, and the energy need (even if efficient) should it be a goal to make it possible for a huge population to buy this and make it hotter for most of the population who cannot buy it? Does it not actually make the problem worse, and take focus off other solutions and approaches (next qn)?

Qn : Is it not better to first not allow the same level of heating to occur, through passive cooling, materials, white paint, smart planting etc? Is it not better to design for better airflow, or better waterscapes, better buildings, better materials, shaded landscapes? Is it not better to have more NMT and public transport to reduce one of the largest heat sources in our urban spaces?

At this point, it is a little worrying how we think of “solutions” without accounting for the dynamics around them, without considering the possibility that we might be worsening the problem we are trying to solve.

Unless I’m reading this really wrong (that’s happened many times, so who knows), part of the solution seems to be to make it more expensive, not less, to buy air-conditioning, and cross subsidize the other, scalable and smarter solutions for a larger population.

Change my mind :slight_smile:

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Also - was in hot Penukonda day before, and the difference under this roof was 5-7 degrees (hay on a tin roof, actually felt cool) compared to other structures.


The problem isn’t the non-availability of natural systems. The question is, given a choice, how many people want to live under a hay roof?

In a hot place like this - many. In fact the couple running this farm for years was from Bangalore and chose to add the hay atop the tin roof the house had by default. There’s also newer materials, reflective paints, other approaches. My point was to not chase/incentivise the solution that could make it worse, effectively.

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I made a low-cost air cooler today. Here is what it looks like:

And this is the tech powering the cooler:

Here are the components:

  1. An electric fan
  2. Khus grass for absorbing water and allowing for slow evaporation
  3. A reused PVC pipe to drip water onto the Khus grass
  4. Bamboo from the farm for the frame
  5. A small water pump to transfer water from a container to the dripping pipe (yet to add this)

An air cooler based on evaporation cooling might not be the best choice in a humid setting, but it works well in dry climates. Also, once I add the small water pump, the manual effort of adding water won’t be needed.

Structural changes when building things from the ground up might be the optimum solution, but stopgap measures might also work well at times. I understand there is still power consumption involved in this setup. Although I have access to solar power in my village, that also comes at an environmental cost. I tell myself smaller steps while making a choice [here, between buying an AC and building the cooler] are sometimes good enough.


Nice hack!

  • Have you measured how this affects the ambient temperature in the room? Temperature drops from what to what in how much time?
  • While the PVC pipe is salvaged, there is no specific reason for it to be PVC, right? That could also be bamboo (with some additional effort, holes drilled)?

Haven’t measured this, but it seems to work better than an off-the-shelf cooler. Assuming this is because the evaporation area is much larger on this. An air conditioner with refrigerants might do faster and more cooling, though.

True, true. I was originally thinking I’ll use a clay pot on top, and water would drip from it below to the vetiver. Can also be bamboo fairly easily. Was saving up on bamboo to make some more of these :sweat_smile:


Too cool @Mohit

By the way, I remember those Air coolers from 30 years back were extremely loud. How does this score in terms of noise?

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Absolutely agree. The issue is to look at how urban housing design and construction, as well as urban planning is driving air-conditioning use. Less than 10% of households in the country currently have an AC. I don’t think that making it more expensive is going to be a strong enough deterrent for this set of consumers.

The only electronic component here is the electric fan. The box in the old coolers acted as an echo chamber for the sound from the water pump & the fan within it. So, far quieter than those :grimacing:

Would love to figure what kind of studies have been done trying to plot out how different kinds of improvements to the built environment has led to better cooling levels and how this compares to air conditioning.

With my anecdotal experience at Himachal I have seen how houses built with stone etc. require fundamentally less heating (read: burning firewood, less emissions etc.) as compared to the concrete houses. There are a ton of second order benefits to this which don’t get captured as easily.

Maybe you can shed light on this @Smita with your experience at Energe-se.

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Evaporative cooling is very well studied and well understood. The Wikipedia page gives a good overview: Evaporative cooler - Wikipedia

There are studies - both energy modelling and actual monitored data, which show that indoor temperatures are much lower than conventional buildings, even during peak summer periods. The building improvement interventions use design and construction materials to reduce heat from coming into the building, and also improving ventilation and shading. This minimises ‘discomfort hours’ and reduces the need for cooling, and also air-conditioning use.

Vernacular buildings like you mention in Himachal, are more comfortable as they are designed to be suited to their climate - something we have stopped doing in urban areas. That said, due to lifestyle changes and the fact that climate change has impacted weather patterns (higher summer temperatures and heat waves that last longer etc.), means that even vernacular architecture needs to be adapted to the new reality. Agree about the co-benefits, they not adequately captured in the silos of climate action.

Would be glad to share more information Rishabh.

Okay, a significant part of this thread intersects with my college days’ research topic: Phase Change Materials (One of many available smart materials). So, I have to jump in! :slight_smile:

First, let me start with my pov: I agree with all the points - AC is not a sustainable and affordable solution for a nation (1.4 billion population) such as ours. In fact, if we are taking this route - we will have to come back to fix all the unintended consequences (2nd order effects) that AC will create. A few are already in front of us.

And there is an abundance of available natural solutions which might be costly initially but at scale will be affordable for almost everyone. I remember working on one such project in my college days

In fact, I have published 7 to 8 research papers on this material for multiple use-cases

If I can recall my research, PCM does has the ability to hold heat without changing its phase this means it can actually absorb excess heat and reduce the temperature at zero energy(electricity).

Now, if we compare its functional properties to reduce temperature - it is the same as an AC, no?
But, PCM has functional and utilization limitations: it can create a maximum temperature Δ of 6 to 7 degrees C and demand some adjustment for existing buildings.

However, if utilized smartly during construction, the results have been really good! Most of the Green buildings have used PCM

Here is the list of some used PCM:

  1. Paraffin Wax - CnH2n+2 (n=20-40)
  2. Glauber’s Salt - Na2SO4.10H2O
  3. Sodium Acetate Trihydrate - NaC2H3O2.3H2O
  4. Erythritol - C4H10O4
  5. Polyethylene Glycol (PEG) - HO-(CH2-CH2-O-)n-H

In fact, inorganic wax is also a PCM but with limited functionality :slight_smile:

Depends on

  1. Urban planning (or lack of as a whole).
  2. Geography/Locale

Almost every tin shack in Dharavi has a window AC unit even if they don’t have running water or sanitation. Given the humidity of Mumbai you will need an AC simply to de-humidify the air and with that remove the heat held by the humid air.

The only way to solve a situation like that would be Urban Planning.

On the other hand passive solutions have a greater impact where there is a modicum of urban planning at least that can promote air flow and landscaping.

Reflective painting + smart planting will yield the greatest passive cooling.

An experiment from my house in South Bangalore (done a year ago, last April), to measure the efficacy of white roofs:


Temperature of Roof Surface (Exterior):

Corresponding Ceiling Temperature (Inside house):

I then laid a white tarp on a section of the roof to reflect the sun rays, as an experiment:


Roof (Exterior) Temperature after White tarp:

That’s a whopping 13°C drop (58.5°C → 45.5°C) in exterior temperature simply due to reflection.

Ceiling (Interior) Temperature after laying White Tarp:

That’s an 8.6°C (47.4°C → 38.8°C) drop!

Interior room air temperature itself dropped from ~34°C to 28°C.

While this didn’t completely negate the requirement for an AC, it did reduce the load on the AC and how frequently I turn the AC on.

My conclusion:
With use of Phase Change Material roofing tiles coated with Solar Reflective Paint, one could eliminate the need for an AC all-together in cities with temperate climates like Bangalore. Add roof gardening into the mix and it could possibly work in other cities too.

The GoI’s National Disaster Management Authority has been pro-active the last few years in trying to prevent heatwaves.

They launched a cool roof challenge ( trying to get cities to implement cool roofs. I’m not sure about the success of the program though.

They did however come out with a handbook for implementation of cool roofs, with ~15 different cool roof tech & designs, the book has some rather good information and is worthwhile for anyone planning to implement cool roofs:


That is quite cool.

If the entire roof was laid with solar panel, that should significantly help reduce the temperature as well?

Something else I came across today relating to this topic.

“If the entire roof was laid with solar panel, that should significantly help reduce the temperature as well?”

It depends on what one is looking to achieve. In the daytime, panels have a higher temperature, so cool roofs are better. It is reversed at night. However, both solutions are preferable at a house level, to standard roofing material.

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Is that a pun? :stuck_out_tongue:

Back to the question at hand:

tl;dr: Covering the roof with solar panels

  1. Will be about ~40-45% as efficient as a cool roof if there are multiple rows of solar panels
  2. Will be about ~80% as efficient as a cool roof if there is a large single sheet of panels


We have to consider the Sun’s azimuth and elevation on the hottest day/week of the year to calculate the % of the roof that will be in the solar panel’s shadow.

Image Source: Understanding Azimuth and Elevation | PhotoPills

In order to achieve maximum efficiency, the solar panel will have to track the sun as the sun traverses the sun path through the day and through the year:

Image Source: How the Sun Moves Through the Sky | Solar Power for Ordinary People

However, such mechanisms are cumbersome and most solar installations just tilt the panel at a fixed angle for optimal efficiency at both summer and winter solstices. For Bangalore at a latitude of ~13° that tilt angle 𝛽=13°, with the solar panel facing south.

Image Source: 2.11 Collector Orientation | EME 810: Solar Resource Assessment and Economics

Note altitude = elevation, those terms are interchangeable.

Case 1: Cooling effect of a large Single Slab style Solar Panel Installation

In Bangalore, the hottest week has historically been the last week of April, with April 28-30 being the hottest day of the year for the last decade or so. On April 29th the elevation of the sun at noon is ~85°. Using basic trigonometry then we can calculate the % of roof impacted by the sun’s rays i.e. not in the shadow of the solar panel.

About 98% of the roof will be in the solar panel’s shadow if we cover the entire roof with solar panels as a single slab of panels at noon.

However, as the sun rises and sets with change in the azimuth there will be some more part of the roof that will be lit by the sunlight and as such heat up.

Accounting for the azimuth and elevation together over the entire day, ~87% of the roof will be under shadow taken over time. Please note that the ~87% is just a quick back of the envelope calculation so in the ballpark.

To verify this visually, I built a quick CAD model of a house, with the roof entirely covered with solar panels, placed at a 13° inclination:

1Angle 1Front

The house model was placed at the geo coordinates close to my house in South Bangalore, and the sun’s path was modelled over it on April 29, 2023 (assumed hottest day of this year) from dawn to dusk. You can observe the sun rays get under the solar panel in the early and later hours of the day corresponding to the azimuth and elevation changes and their impact on the roof:


Now with only ~13% of the solar radiation impacting the roof (as compared to no full-roof-solar-panel), and accounting for sunlight scatter from nearby buildings, and heat conduction from the panel itself to the roof, I’d safely conclude that covering the roof with a solar panel could be about ~80% as efficient as a cool roof.

Case 2: Cooling effect of traditional multi-row Solar Panel Installation

However, in traditional installations solar panels are divided into multiple rows, due to limitations of panel size and cost, weight, and size of the supporting structure.

Space is left between these rows to allow sunlight to impact the bottom of the 2nd row of panels when the sun is at its lowest elevation on the Winter Solstice (Dec 22).

Image Source: Solar Photovoltaics | Impact of shading

I modelled the same house with two panels to show how the gap allows for the second panel to get as much sun on winter solstice when the sun is at the lowest elevation:


I wanted to compare a traditional solar installation w.r.t. the idea suggested by Nithin of covering the full roof with solar panels (Case 1, above)

Modelling the same house with two rows of panels:

2Angle 2Front

Again, the house is placed in South Bangalore and the sun’s path was modelled over it on April 29, 2023 (assumed hottest day of the year) from dawn to dusk. You can see that a much larger area of the roof is impacted by the sun’s rays compared to the previous case of covering the full roof with panels:


A back of the envelope calculation shows that only around ~62% of the roof is under shadow throughout the day during peak hours and ~44% when accounted for throughout the day.

However, for the same house placed in Delhi, with two rows of solar panels, nearly 70% of the roof is under shadow on Delhi’s hottest day (mid may usually ~May 22th) during peak hours, but when accounted for throughout the day the shadow area falls to <50%:


Now with ~50% of the solar radiation impacting the roof on average, and accounting for scatter from nearby buildings, heat transfer from the panels itself etc. I would conclude that in case of multiple rows of solar panels the design would be only ~40-45% as effective as a cool roof.


amazing! thanks for sharing!