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Knowledge Centre – Passive Strategies

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A desiccant is a substance, either solid or liquid, which absorbs water molecules from air and dehumidifies it. The desiccant, initially used to absorb moisture from the air, is later regenerated by heating the desiccant so that it releases the absorbed moisture. This phase change cycle is a continuous process that drives the operation of desiccant systems.

Two basic categories of desiccant systems are:

  1. Open desiccant systems where desiccant comes into direct contact with the air for the process of dehumidification.
  2. Closed desiccant systems where desiccant is confined to a closed chamber and dehumidifies air indirectly.Based on the type of desiccant used, desiccant systems can be categorized as solid and liquid desiccant systems. In solid desiccant systems, a dry solid desiccant like silica gel or zeolite, is used in a rotating bed or impregnated into honeycomb-form wheel within the system. Liquid desiccant systems are a new emerging technology consisting of a contact surface, which is either a cooling coil or cooling tower, wetted with liquid desiccant like lithium chloride or calcium chloride.Desiccant systems can be successfully used in regions with low heating demand but have limited application in high humidity areas where desiccant fails to reduce the air moisture content to desired level. In India, no commercial project is functioning on the concept of desiccant cooling. Research projects are underway to make the application more viable.

Table 1 Advantages and disadvantages of desiccant systems

Pros Cons
Improved air quality in interiors High initial cost to setup the system
Less electric consumption as alternative energy sources can be used Experienced professionals required to construct and service such systems
CFC, HFC, HCFC refrigerants are not used Liquid desiccant could be corrosive and damage the system or components
Integration with conventional systems to remove latent heat load can reduce energy consumption Cost effective only when there is a source of waste heat available to regenerate desiccant
Low operational cost
Case study:

In India, no commercial project is functioning on the concept of desiccant cooling. Research projects are underway to make the application more viable.

Resources and tool
  • Desiccant Cooling with Solar Energy by CIBSE
  • An assessment of desiccant cooling and dehumidification technology by Oak Ridge National Laboratory
  • solair-project.eu
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Evaporative cooling is a mechanism traditionally used to provide thermal comfort in hot and dry regions. The mechanism involves sensible and latent cooling of air with water. Direct evaporative cooling is most effective when the outside condition is dry and below the desired conditions. Indirect evaporative system is used during the seasons when little or no humidification is required i.e. when outside air humidity is within a comfortable range. Fresh filtered air is made to pass through a dry section of the system to cool the air through sensible heat transfer. Stage wise evaporative cooling systems can be either two stage or three stage.

  • Two stage evaporative cooling systems (direct + indirect) – the direct system could be functional during the dry season, when humidification of air is required, and indirect system can be used when air primarily needs to be cooled.
  • Three stage evaporative cooling system (direct + indirect + cooling coil) consists of direct and indirect evaporative cooling together with conventional cooling coil. The addition of cooling coils (chilled water or refrigerants) is helpful in monsoon season when the humidity level is high and dehumidification is required. Fresh air passed through the coils controls both sensible and latent heat requirements. The coils are also useful in winter season when some heating is also required.

The drawback of the two stage system is the high humidity level of the supply air. Over a period of time indirect evaporative cooling systems which provide sensible cooling of the air without humidification have emerged in the market.

Table 2 Advantages and disadvantages of evaporative cooling systems

Pros Cons
Can be added to existing chilled water systems at low costs Needs high purity water to prevent the build-up of salts
Reduces use of HFC refrigerants Requires periodic maintenance
Reduces energy costs significantly
Case study:
Case study Central University of Rajasthan
Location Bandar Sindri, Ajmer, Rajasthan, India
Climate Type Hot and dry
Building Type Residential
System Description Two stage evaporative cooling
  System consists of a direct evaporative pre cooler which provides cool and wet air to indirectly cool down the primary air in the tube bundle heat exchanger. The cool and dry air is then passed through a direct evaporative cooler to humidify it.
System Performance Energy consumption in the hostel building is estimated to have been reduced to 1/3rd of a similar building with no major energy conservation measures and using conventional air-conditioning systems. Indoor temperatures were measured to be between 31 °C to 34 °C when the ambient was approximately 44 °C.
Energy Performance Index was measured to be 60 – 65 kWh/m2/year (2012)
Resources and tools
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Cooling loads in tropical countries like India peak during the hot summer season when solar radiation is available in abundance. Thus, application of solar cooling technology uses a renewable source of energy to reduce the cooling loads when air conditioning demand is at its annual high. Principle behind the functioning of solar cooling is the use of solar heat/ thermal energy to re-generate the refrigerant in absorption chiller or desiccant in a desiccant chiller.

Absorption chillers comprise of the following components:

  1. Evaporator: where the refrigerant evaporates at a very low pressure and temperature and is absorbed by the absorbent. The process results in extraction of heat from the refrigerant and provides chilled refrigerant as an output.
  2. Generator: The mixture of absorbent and refrigerant is then introduced in the generator. Steam or hot water produced through the solar panel devices is used to vaporize refrigerant.
  3. Condenser: The vaporized refrigerant will be cooled down in condenser and maintained at low pressure. This cooled refrigerant will be further used in evaporator for generation of chilled water for air conditioning.

Table 3 Advantages and disadvantages of absorption chillers

Pros Cons
Attractive payback when configured with power generation and hot water heating Requires significant space for the solar panels and solar concentrators; thus suitable for large projects only
Low distribution losses in the range of 5 to 10 percent; conventional technologies is between 75-80 percent Cost almost twice as much as conventional chillers
Eliminate the use of CFC, HFC, and HCFC refrigerants Requires greater pump energy compared to electric chiller
System operations generates less noise and vibration Higher flow rate of condenser water  required as absorption chillers have lower COP’s
High efficiency in triple effect absorption designs Requires large cooling tower capacity compared to electric chiller as larger volume of water is circulated
Case study
Case study Solar Energy Centre
Location Gurgaon, Haryana, India
Climate Type Composite
Building Type Office and residential
System Description Solar air conditioning
100 kW cooling capacity standalone system is integrated with triple effect Vapour Absorption Chiller (VAC) and solar parabolic concentrators. The system is designed to meet cooling loads of 13 rooms at the centre. VAC can use steam, hot water, gas, kerosene or oil to run continuously. Chilled water is supplied at 7 °C through FCUs to all rooms. Solar collectors, with area of 284 sq. mt., deliver water at temperatures between 140 °C and 210 °C.
System Performance The integrated system is estimated to be 20 percent more efficient than VACs with no solar component. Co-efficient of Performance (COP) is 1.7, which is highest among the vapour absorption technology coupled with eco-friendly energy resource. System has built-in thermal energy storage using phase change materials. This allows it to supply cooling continuously.
Resources and tools
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In a developing country like India, high volume of waste heat is generated around the year which could be used to produce heat around the year. Thus a technology which uses thermal energy to provide cooling could be a solution to our rising energy crisis. In many buildings like hotels, hospitals, and industries, there is a demand for hot water along with air cooling. Such a scenario is well suited for the application of Tri-generation concept.

A tri-generation system produces three forms of energy, i.e. electricity, heating, and cooling which could be used to generate power, hot water, and air conditioning with suitable equipment. The principle of tri-generation is based on the generation of heat energy. Heat captured through burning waste, production of electricity with generators, or heat generated through solar panels could be used to generate hot water through heat transfer equipment or cold/ chilled water with absorption chillers. Potential for using tri-generation systems has been identified to be nearly 500 to 1,000 MW in India.

Tri generation technology, also known as Combined Cooling, Heating, and Power (CCHP), comprises of a gas engine or a power system operated by burning waste, bio fuel, or fossil fuel to produce electricity. The connected heat recovery system is used as a heat exchanger to recover heat from the engine or exhaust. This recovered heat can be used for heating applications like hot water or a regeneration process in absorption chillers. The electricity produced within the tri-generation process could be used to meet the building loads or power chillers during peak load period.  The thermal energy could be diverted to boilers to heat the water used in hospitals, hotels, and industries for numerous purposes and/ or to absorption chiller to heat the absorbent and refrigerant mixture and regenerate the absorbent.

Table 4 Advantages and disadvantages of Tri-generation systems

Pros Cons
No or reduced use of CFC or HCFC based refrigerants Intense planning and research is required for designing systems for projects
Tri generation system’s efficiency is 90% in comparison to 25% for electricity produced in power plants Applicability differs with each project; detailed feasibility studies are required for each new project
Huge benefits in terms of reduction of GHG emission (estimated at over 50% from the current GHG emissions levels in buildings having mix of cooling and heating load)
Same equipment can be used to generate cooling, hot water and electricity in a building
Has potential to double gas  end user productivity (compared to combined cycle power station)
Case Study
Case study Pushpanjali Cross lay Hospital
Location Ghaziabad, India
Climate Type Composite
Building Type Hospital; 400 bed tertiary care
Area 5000 sq. mt.
System Description Tri generation system
1000 TR air conditioning load. Components to meet the heating and cooling loads include a gas genset (1.7 MW), 600 TR capacity Vapour Absorption Machines (VAM) with heat recovery, and electrical chillers of 400 TR capacity.
System Cost Total capital cost including DG backup was 9 crore INR. Additional capital investment for the Tri generation system was nearly 3.4 crore INR. Cost of power generated through the Tri generation system (using natural gas) is 3.4 INR/ kW. Net savings of 3.8 INR/ kW or approximately 3 crore INR annually is achieved through this system.
System Performance The system provides uninterrupted and reliable power supply, without any fluctuations to the hospital. Power supplied by natural gas is more environmentally friendly than the coal based power supplied through the grid. Operating cost of this system is 1.36 lakhs INR per day compared to 2.25 lakh INR for using electrical chillers running on grid supply.
Resources and tools
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Guiding principle of a conventional air conditioning system is convection whereas in a radiation system, the guiding principle is heat transfer through radiation. Heat transfer predominately occurs through surfaces like floors, ceiling, or wall which in turn are heated or cooled by embedded coils. Radiant systems are installed in combination of large thermal mass to facilitate absorption and radiation. For optimizing performance of the systems, coils should be installed in floors for heating purposes, and in ceiling for all cooling purposes. Application of radiant systems is limited to areas which have high latent load and chances of air leakage from humid areas are high. Improperly installed systems can lead to condensation on the building structural elements.

Types of radiant cooling

  • Chilled slabs: These deliver cooling through the building structure, usually slab, and are also known as thermally activated building systems.(Figure 5)
  • Ceiling panels: These deliver cooling through specialized panels.

 

Systems using concrete slabs are generally cheaper than panel systems and offer advantage of the thermal mass while panel systems offer faster temperature control and flexibility. Capital expenditure of this system is the same as a high efficiency chilled water system; however, operational expenditure is less than the chilled water system.

Radiant cooling systems consist of coils embedded within the structure. These coils carry chilled water generated either through conventional electric chiller systems or low energy chilled water generation systems like absorbent chillers, desiccant chillers. Chilled water in the coils cools down the slab or panels which in turn act as heat sinks for sensible heat loads of internal spaces.

Concrete structures typically used with radiant cooling systems also increase the thermal mass of buildings.  This introduces inertia in the structure against temperature fluctuations and allows it to absorb heat from internal spaces.

Table 5 Advantages and disadvantages of radiant cooling systems

Pros Cons
With a ventilation air system, thermal mass can significantly reduce the need of air side systems reducing the fan power in HVAC system drastically Sections with leaking or blocked radiant pipes have to be closed, disrupting supply in the process
Noise and drafts of air movement are removed. There are no diffusers in the way of décor and cleaning Condensation reduces cooling capacity. Hence an efficient envelope with non-openable windows is required.
Additional savings due to lower supply temperature of chilled water (about 7-9 °C lower) Condensate formation on the cold radiant surface results in water damage, moulds etc.
Better comfort conditions are maintained inside the space Complicated controls required skilled maintenance staff
Not easy to maintain temperatures below 23 °C
Case Study
Case Study Infosys Software Development Building (SDB-1)
Location Hyderabad, India
Climate Type Hot and dry
Building Type Office
Area 11,600 m2
System Description Radiant cooling system
  The two symmetric half of the buildings are air cooled with different technology. A conventional cooling system is implemented in one half whereas the other half is cooled by radiant system. Radiant system was designed for a cooling output of 75 W/ m2. Chilled water design temperatures for supply were 14 °C and for return 17 °C. Cooling tower approach temperature is 2 °C. Low pressure piping and ducting distribution system is installed. Energy recovery is also installed to provide dehumidified air to offices.
System Cost Capital cost of the radiant system was 3,302 INR/ m2Conventional system was installed at 3,327 INR/ m2
System Performance Energy index for radiant system was 25.7 kWh/ m2. compared to 38.7 kWh/ m2. Thus, efficiency of radiant system was 33% less than the conventional system. Average chiller plant efficiency for the radiant system side was .45 kW/ TR compared to .6 kW/ TR.Quantity of water required by the radiant system is one fifth of that required by a conventional chiller of similar capacity.Air quality is also improved as there is no recirculation, so contamination is reduced. Comfort conditions measured inside the building are within the permissible limits of ASHRAE 55-2004 and ASHRAE 62.1-2007 for most of the time.Building has been operational for the last 3 years and the radiant system is functioning without any major complaints.
Resources and tools
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A Ground Source Heat Pump (GSHP) system heats and cools building by using earth as a heat source or heat sink. The system either extracts thermal energy out of the ground or transfers thermal energy from buildings to the ground. Moreover, heat energy stored during the summer season could be extracted out during winters to heat the ambient spaces. On average, 46% of the total solar energy received is stored. At 4-6 meters below ground surface[1], temperatures are more or less constant. Heat could be pumped in during summers to the ground, where the temperature is lower than the ambient temperature. GSHP system has three major components:

  1. Earth Connection

Earth connection is the connection between the GSHP system and the soil.  Most popular connections are tubes, introduced either horizontally or vertically into the ground, or submerged in a lake or pond. The tubes carry an anti-freeze mixture and a suitable type of heat transfer fluid.

  1. Heat pump

Heat pump helps heat transfer from fluid in the earth connection to the distribution system. The heat pump consists of a heat carrier like water or air, which absorbs heat from heat transfer fluid through indirect contact and subsequently carries this heat energy to the heating/ cooling distribution system. In a reverse cycle, the heat carrier transfers heat from the distribution systems to the heat transfer fluid in the earth connection.

  1. Heating/ cooling distribution system

This system delivers the heating or cooling from the heat pump to the ambient spaces. It consists of air ducts, diffusers, fresh air supply systems and control components, and circulates the supply air as per design conditions and occupants requirements.

Table 6 Advantages and disadvantages of GSHP systems

Pros Cons
The system has high EE  (50 – 70%) and provides heating, cooling and hot water More suitable for composite climate as continuous pumping of heat to ground could make the ground saturated
Low maintenance requirement as well as operates at a lower cost It requires an intense concept design stage research to check the feasibility of the system on the individual site
Uses renewable source of energy Requires skilled labour for designing and installing the system
  Government approval may not be available for all states
  High initial investment
Case study
Case Study Central University of Rajasthan
Location Ajmer, Rajasthan, India
Climate Type Hot and dry
Building Type Residential – student hostel
System Description Geothermal hybrid cooling system
  Water from the boreholes is used to cool the air before introducing the same in the indoor spaces. Heat exchange between water from boreholes and supply air takes place inside specially designed AHUs. Air is further cooled after this through a two stage evaporative cooling system. The system uses 100 percent fresh air and is designed to reduce temperature of the supply air by around 10 °C. Design temperature for the hostel rooms is 29°C.
System Performance Average outlet water temperature from the boreholes is recorded at 25 °C. Outlet temperature in winter is around 19 °C.
Energy Performance Index was measured to be approximately 78 kWh/m2/year (2012).
Tools and resources
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