Put This Energy Hog On A Lean Diet

Air conditioning consumes most energy in a household. The chart (Yearly Energy Bill of a typical flat at Mumbai… ) gives an idea of how the energy bills increase because of using Air Conditioning machines at home. Thus, in today’s situation, we need to be more careful while using ACs at home or elsewhere…

Menu for A Lean Diet for the Energy Hog

A. Reduce load
B. Use star rated equipment
C. Use low energy appliances
D. Maintain well.

We shall consider load reduction methods only as under:

A. Solar Load

• Structure cooling
• Radiant barriers
• Low transmittance glazing
• Thermal barrier/reflective paint for walls and roof
  Night sky cooling.

B. Fresh Air Load

• Wind towers
• Indirect evaporative coolers
  Exhaust air recuperators

C. Internal Loads

• Use low energy lighting and machines
• Isolate hot spots and vent them directly
• Wear light clothing.

this article will discuss structure cooling and fresh air load reduction only.

Solar Load Reduction Methods

Structure Cooling

There are many ways to do that, but the best way is to keep the structure cool by using the techniques used by our ancestors. Our heritage buildings remain naturally cool even today without using any energy.

While they used massive construction as thermal storage to absorb and water bodies to drain out the solar heat, we can’t use that technology any more in modern times, due to space and cost constraints. This article will examine in detail the various modern methods based on our Heritage techniques to achieve the same result.

1. A Simple Method suggested by CBRI Roorkee.

Prof. S. P. Jain devised a method where a layer of gunny bags was laid on a roof and wetted frequently. The water absorbed the heat from the roof while evaporating. He claimed that about five litres of water could produce one ton of cooling. He called it Roof Surface Evaporation Technique. A hand pump replaced the lost water.

2. An improved version had a coir mat, an electric pump with timer, and garden sprinklers. In both the versions, the terrace was not usable and there were waterproofing issues.
3. The author devised a system of welded iron pipe grids filled with water and connected to a unit that was an evaporative cooled heat exchanger.

The entire system was under vacuum, so the water boiled at 24 Deg.C. and absorbed the solar heat from the slab. The vapour travelled to the heat exchanger and the condensed water returned to the grid by gravity. The pipes were laid on a cured concrete surface and covered by screed.

The system kept the roof bottom below 30 Deg.C in Jaipur in mid-summer. Here the issue was the need for highly skilled welders in the field. Also, it would be impossible to repair a leak if it developed after the screed was applied. However, the Jaipur system is still working after almost twenty years.

4. The current versions use corrugated polypropylene tubes in one or more serpentine loops, each a 100 meters long, connected to supply and return manifolds. They are laid on the surface of a new or renovated concrete roof, tested for leaks and then covered with screed. A special fitting allows repairing of leaks.

Dia 1. Experimental setup at Mumbai…

Dia 2. Multi storey system at an office at Nashik

Dia 3. Floor and roof cooling system for a museum at Dholka, Gujarat…

Dia 4. Piping layout for Veer Savarkar Smarak at Mumbai…

Dia 5. Wind tower at G. B. C. at Hyderabad…

Dia 7. Triple mode cooler with radiator, cooling pad and chiller…

Dia 8. Indirect evaporative fresh air cooler…

A low pressure pump draws water from a tank and circulates it, via the manifolds, slowly through the loops, so that it returns very hot. The way in which the return water is treated before it returns to the tank gives rise to several variations.

1. Variant 1.In this simplest system, the hot water is dumped directly into an underground tank that acts as thermal storage for the day’s collection of heat from the roof. It mimics the massive heritage walls. Water can hold twice as much heat by volume as masonry can for the same temperature rise. This heat is dissipated into the ground by conduction.

This system is viable for very small structures; otherwise the tank becomes too large.

2. Variant 2.In order to reduce the underground tank size, the hot return water passes through a fan cooled radiator. Thus, most of the heat is dumped into the air and only tepid water enters the tank. Even this heat is removed by again circulating the water through the radiator at night. (Dia. 1)
3. Variant 3.When space constraint or rocky ground prevent an underground tank, then a small tank is placed below a larger radiator. This works well in mild climates like Mumbai.
4. Variant 4.In hot zones, an evaporative cooling pad, with its own tank and pump, is added before the air enters the radiator. A thermostat activates this only if the ambient temperature rises above say 30 Deg. C. It shuts off below the set point, thus saving water. (Dia.7 )
5. Variant 5.Multi story application. Here, an insulated overhead tank supplies cooling water to each floor via a downpipe feeder, controlled through an inlet valve. However, in order to prevent the build up of high pressure in the plastic pipe, the discharge end has no valve. It is connected to a vertical return pipe through a vacuum free connection. The vertical pipe connects to an underground tank. The radiator unit is placed outside at ground level. The cooled water is supplied to the overhead tank, when required, by a pump. (Dia. 2 & 3)
6. Variant 7.If a more precise control is desired, a small chilling unit can be added to any of the variants. (Dia 2 & 7). A little goes a long way here, A one TR unit maintains the bottom of a 3000 sq. ft. at 30 Deg. C. at one installation at Mumbai. (Dia. 4) The danger is that if the structure is cooled below the ambient dew point, then there will be condensation on it.

This system is neither a jugad nor a cheap copy of imported systems. It has been developed in over twenty years of research as to how our own Heritage buildings remain cool without any equipment. Several undergraduate and two Master’s level research has been done in our own pilot unit. The zero energy concept of thermal storage and drain is not found in any of the imported systems. By adding a small P.V. system, our system becomes a zero impact one.

All materials are made in India. The system is simple and affordable enough to be replicated by anyone with basic technical skills. There are no patents or copyrights.

Fresh Air Load Reducing Methods

1. Wind Towers with thermal storage:–Fresh air load can be greatly reduced in hot dry climates by an innovative use of Wind Towers. There are two variant designs. Both are based on the wind towers used in some Middle Eastern and European countries, but they are stuffed with impervious thermal mass that is evaporatively cooled at night. During use, the thermal mass absorbs the heat from the incoming fresh air and greatly reduces its load. The two towers are:

• The wind tower at GBC, Hyderabad: There are two 15 metre tall towers, stuffed with concrete hollow blocks. Their holes are aligned so that air and water can floe downwards through them. Water is sprayed only at night. During the day the dry blocks perform sensible cooling before feeding to the AHU. The total load reduction was 80 TR. for thr two towers.(Dia 5)
• An improved design at Shrujan Centre in Kutch: Here, the concrete blocks were replaced by water filled plastic bottles. They were placed in crates and were simply stacked. Water and air entered the top. humid air went outside at night. Dry cool air was fed into the ducts during the day. There is no air conditioning. Water bottles are much cheaper and lighter than blocks. Also, water can hold twice as much heat per degree than masonry for the same volume and weighs less. (Dia.6)

Dia 6. Wind tower with plastic water bottles at The Shrujan Centre Kutch…

2. Exhaust Air Recuperators

These devices are heat exchangers that pre-cool the incoming hot outside air by cold air being exhausted out. They are quite useful for applications that require large percentage of fresh air, such as auditoriums and operation theatres. There are two types.

Rotary wheel type has media that absorb sensible and latent heat from incoming air in one section and give it up to the exhaust air in the other section.

• Heat pipe type uses evaporative pads and heat pipes to do the same. However the two air streams are totally isolated.

3. Indirect Evaporative Pre-Coolers

Both use heat pipes and evaporative pads. The two stage design cools more, but uses more primary air and water, (Dia. 8)

Conclusion

If the commitment made by India at Paris is to be honoured, then there is an urgent need to reduce the energy consumption by air conditioners since they have the lion’s share of the energy budget of every building. The simple, effective and affordable solutions described in this article use the time tested techniques of our ancestors and modern technology developed and validated in India that uses only materials made in India, can go a long way towards achieving our goal if implemented on a mass scale.

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