SOLAR AIR CONDITIONING
Large size Air Conditioning plants for commercial buildings, hotels, hospitals, air ports and industries are conventionally operated through one of these energy sources,
a) Electric Power, either through grid or in house power generation
b) Steam from boilers
c) Hot Water from process wastes
d) Direct gas operated chillers
Hot Water operated chillers require water temperature of 85̊ C to achieve optimum performance. At this temperature they provide up to 0.8 COP. These chillers operate on absorption principle. LiBr is used as absorber and water as refrigerant. LiBr has strong affinity for water vapors when heated. This chemical activity is used to chill water required for cooling of the building. No compressor like conventional chillers is required.
Hot Water is generated through solar thermal technology. Solar photovoltaic cells produce dc current which is stored in gel batteries and converted to AC through converter. While thermal tubes produce hot water. This is directly used to get cooling effect with absorption chillers. Small capacity chillers for residences down to 4 tons use adsorption technology in place of absorption. Adsorption system is based on dessicant wheels which are regenerated through solar hot water. These wheels dehumidify air to produce cooling effect. This is all air system. No chilled water is produced. Room air is cooled by passing over bank of dumidifiers and hot water provided to charge these.
For commercial application absorption chillers with LiBr/water cycle provide maximum efficiency. With this system issue arises about operation of cooling plant after daylight hours. For this purpose storage vessels are used. Large capacity tanks are filled with water during sun hours. These are large enough to keep chiller system operational during night time. Tanks are insulated with low U-value thermal insulation. Insulation R-value is selected keeping in view ambient conditions and block heat loss for minimum next 24 hours.
High efficiency flat plate or parabolic concentrating type evacuated tubes are used for thermal collectors. Small amount of electric energy is required to operate water circulating pumps for hot water, condenser water as well as chilled water. Chiller refrigerant pump and absorber pump also require electric energy. Cooling Tower fans are electric driven. These electric demands are met through photovoltaic cells to make complete HVAC plant based on solar energy.
Limiting factor is the space required to place Thermal Tubes collectors on roof top or at ground. Generally 150sft of flat roof or ground area is required for placement of collectors to provide one ton of cooling .
Using this technology World’s largest solar thermal A/C system of 450 ton capacity chiller plant is installed in Singapore at UWC-SEA. This is designed, installed and operated by Austrian company SOLID GmbH, Graz. System is in operation since October 2011.
Building Insulation R-Value, Compressive Strength,Water and Fire Resistance
Building Insulation provides effective tool to reduce energy costs for the building. Insulations are used for roof, walls, below grade, pipes, chimneys, attic spaces and ducts. Before selecting insulation, clear understanding of pros and cons and selection criteria is helpful in decision making and reducing initial costs.
For any insulation application major considerations in addition to price factor are as follows,
a) Insulation R-Value
b) Water Permeability
c) Compressive Strength
d) Sound Reduction
e) Fire Resistance
Insulation R-Value is the prime criteria and reflects the main purpose insulation is used in the building. Its value determines how much resistance particular insulation will provide to heat flow. Insulation is required in a building to minimize flow of heat to and from the building. In summer due to high ambient temperatures outside , direction of heat flow is from outside to inside. As inside temperature is maintained close to 74̊ F, any temperature above this value will cause heat to flow towards space being cooled. In the absence of building insulation cooling energy is wasted. Likewise heating costs will be much higher without insulation in winter.
How much insulation is required is depicted by required R-Value. Structural components like concrete slabs or brick walls provide very little resistance to heat flow. Some type of insulating material is added to outside building envelope components to enhance R-Value. Say roof elements, slab, false ceiling, screed, and tiles provide R-5 value and we want to arrive at R-25. Insulation equivalent to R-20 will be added.
One inch of Rigid Polyurethane board provides insulation equivalent to R-6. Similarly one inch of extruded polystyrene provides R-5 value. Safety factors are added to select required thickness. Tables provided by manufacturers are based on lab testing conditions. In real world we have number of variables like wind speed, prevailing mercury reading, traffic and last but not the least aging affect.
Every type of insulation degrades with passage of time. Thermal conductivity (u-value) increases. Some chemicals used during manufacturing process evaporate after some years replaced by air particles. These particles reduce insulating value eight to ten years down the road. Due to these reasons appropriate safety factor is added at the time of selection so as to remain within range during whole life span of the building.
Water absorption protecting layer is applied on both sides. This may be aluminum foil, polyethylene sheet or fiberglass coating. This layer will mitigate water transportation to the building.
Fire resistant and non smoke development features are other parameters critical to interior application of these materials. In the event of fire at some remote location, non fire rated insulation may spread this fire and provide continuous channel to every nook and corner where this is applied.
Density of applied insulation determines compressive strength required to bear load of flooring and traffic on top of it.
OPTIMUM CHILLER PLANT SEQUENCE OF OPERATION
In order to achieve energy efficient operation of chiller plant, a computer based supervisory controller is utilized. This controller receives input Data from sensors located in pipe lines and outside. Supply and Return Temperatures, pipe line pressures and chilled water flow rates are available. This data is utilized by the controller for calculating actual cooling demand in the building, and for sequencing of the plant. Plant includes all chillers, boilers, chilled/hot water pumps, condenser water pumps, cooling tower fans and variable frequency drives.
Following sequence may be used for highest energy efficiency.
1) Set point temperature for chilled water is entered to the controller. When temperature is higher than set point, chiller plant is started.
2) Integrated Part Load Values for equipment are determined and data of energy consumption at part loads for each machine is fed to the controller. Building actual cooling demand at any part load condition is calculated by the controller by taking flow reading and chilled water delta t. Controller decides which set of equipment will provide best performance at any load condition. These equipment are brought in to the circuit
3) Leaving chilled water temperature at the chiller is very critical parameter as for as energy consumption is concerned. When chiller is required to produce chilled water at lower temperatures, it consumes more energy. At higher chilled water out temperatures, chiller energy consumption is substantially low. Why temperature is set at higher level or lower level depends upon cooling conditions required. At any stage controller analyses required cooling capacity with optimum chilled water out temperature which is most energy efficient. Chiller is operated at this temperature out condition.
4) Chiller operating efficiency depends on condenser water inlet temperature. Condenser water is cooled at cooling tower. This temperature may be controlled by,
a) Modulating speed of cooling tower fan
b) Flow of water to the towers.
Ambient wet bulb temperature determines up to what condition cooling water may be cooled. Cooling tower water temperature is adjusted by the controller to get maximum efficiency by the chiller. Inlet conditions are varied in different seasons to get better operating conditions.
5) Chilled water is circulated throughout the building by pumps. These pumps consume lot of energy. As cooling demand is not constant in the building, water flow rate to the building is varied accordingly. Variable Frequency Drives on the pump motor modulates its speed. Consequently energy consumed at part loads is lower as compared to peak demand. This step saves energy wastage.