This solar air conditioning case study pertains to the sizing of an indoor solar conditioning system that uses an absorption chiller unit operating with water and lithium bromide (Li Br) for a hypothetical 3 bedroom Villa or house.
Input Data for Sizing:
Climate data for the calculation of solar radiation related to a selected hot climate city. The data relates to a hypothetical two-family dwelling (or large apartment) and to the building’s energy performance as assumed below.
|Indoor conditioned volume||562,5 [m3]|
|Net surface area||187,5 [m2]|
|Maximum heat loss||100 [kWh/day]|
|Month||Average daily |
Design Stage for Solar Air-Conditioning Case Study:
The sizing calculations of the absorption chiller and the solar field was determined taking into account a daily design energy demand due to heat loss of approximately 100 kWh.
Starting with a design value of 100 kWh/day, equivalent to approximately 340 kBTU/day, about 6 hours of operation of a lithium bromide absorption chiller with a calculated cooling capacity of 57.8 kBTU/hr (17 kW).
To generate this cooling power, the unit requires approximately 85 kBTU/hr of intake thermal energy, equivalent to 25 kWt, hence with a yield factor of approximately 0.7.
The fluid (water) needed to provide this heat energy must operate at an absorption chiller inlet temperature that ranges from 70°C to 95°C (standard value of 88°C) and a flow rate of ~1.2 l/s.
In making the engineering calculations for this solar air-conditioning case study, it was assumed that the solar cooling system is without actual thermal storage. Instead, it has a small tank that performs the function of decoupling the solar panel array circuit flow from the intake to the absorption chiller and in doing so reduces any temperature oscillation in the branches of the two circuits.
Nonetheless, it is advisable to design for irregular solar radiation and hence temperatures that are not always constant. That said, it is advisable to reconsider and calculate the yield of the device: hence output is estimated at 0.6 (reduction of the nominal value by 15%).
The thermal power to be supplied to the chiller is then 28 kW. With a daily period of operation of 6 hours, the thermal energy to be produced by the solar panel array is 168 kWh/day.
For this case study of solar air-conditioning, it was taking into consideration the fact that the annual average of daily energy produced per every m2 of solar panels is 2.45 kWh/m2, the total collector aperture area required is approximately 68 m2, of solar thermal panels.
These solar panels deliver thermal energy to the absorption chiller, which generates the cooling power needed to satisfy air conditioning requirements.
As the instantaneous power generated by the unit is greater than the heat loss of the building, the system will have two collection tanks for collecting surplus chilled water generated by the refrigeration system. These tanks will serve to decouple the user from the operation of the chiller unit, guaranteeing sufficient level of cooling power throughout the day.
The proposed design calculations for the solar air-conditioning case study is made up of systems able to provide the energy needed for residential air conditioning requirements throughout most of year. Generating the energy needed to condition the indoor environment based on required thermal loads is possible at any time of the year.
Indeed, during the summer months the system is able to deliver greater cooling power thanks to increased insolation and thus is better able to respond to the higher cooling demand caused by the increased need to condition the indoor environment.
Similarly, during the winter, given the drop in the energy produced by the solar panel array, there is also a decrease in cooling load because of lower outdoor temperatures. Even at this time of year, the system is able to satisfy most energy needs required for conditioning the indoor environment.
A few values specific to the availability of cooling power throughout the year can be found in the table below.
Because solar radiation data relate to average daily values over a month's period, an auxiliary system is planned for days on which solar radiation is below average and the energy accumulated in the collection tanks has already been entirely depleted.
The auxiliary system in this case study is made up of traditional heat pumps. The model selection for these units must be calculated in accordance with the cold distribution system that the application is designed to have.
Air Condition System
The “all-air” system is designed to use heat exchange batteries between the water generated by the absorption chiller and the water generated by a traditional heat pump collected in tanks, and the air that will be subsequently distributed through the indoor environment by a system of ducts and vents. The system described is fully centralized.
For such systems, the devices typically used are “water” heat pumps that work on same accumulations from the absorption chillers, exploiting to the utmost the contribution supplied by the solar cooling system.
Just like an all-air system, a water system is designed for a centralized system where, however, unlike an air system, the chilled water is carried to each room by water pipes, a radiant-type system (floor, ceiling or wall) or thermal convectors that generate a refrigerating effect in each individual room by means of heat exchange batteries.
The thermo-hydraulic industry is well familiar with the different advantages and disadvantages of these two systems, which are briefly discussed below.
All-air systems provide more efficient centralized temperature and humidity control. However, unlike water systems, all-air systems require larger distribution ducts; in addition, there is more complexity involved in controlling the different temperatures inside each room, where it becomes a challenge to regulate different temperature levels.
In water systems (for example: in-floor or ceiling convectors), the temperature of each individual room can be more easily regulated because generation of chilled or conditioned air is demanded of local generation systems that can be effortlessly controlled by the user. However, this type of system requires a distributed generation system that, on average, is more costly.
In such systems, the regulation of humidity is highly complicated. Central conditioning systems using water have a heat reading system that is simpler and more reliable than that of all-air systems.
Because of the heat reading and control characteristics for each room, such systems are more suited to two-family dwellings such as in the case study of solar air conditioning, where it may be necessary to meet the needs of the different users and to share the costs of operation.
Conversely, a combined system is the most costly of all because it requires both air and water distribution/generation systems. However, such a system provides the best of both technologies.
The most commonly cited example is that of a system that is not fully centralized. An example of a combined distribution system is provided by any system designed to generate chilled water by a solar cooling system and to integrate with a traditional split conditioning system.
In such cases, systems have the advantage of making each room completely independent, both from the standpoint of cold generation (selection of air temperature levels) and humidity control, which is fully managed by a traditional air conditioner.
The disadvantage of such systems, in addition to being more costly, is associated with the inability to fully exploit the solar contribution on overcast days. In fact, in a system that is not completely centralized the temperature of the chilled water is not sufficiently low for heat exchange (with air, for example) on overcast days and therefore the contribution of the solar cooling system is not distributed. In such cases, conditioning is provided solely by the traditional system.
This type of solution is well-suited to preexisting dwellings where the traditional air conditioning systems discussed above are already in use.
- Solar thermal panels
- Hydronic control system
- Hydraulic decoupler
- Absorption chiller
- Cooling tower
- Chilled water collection
- Traditional heat pump
- Batteries for generating refrigerated air
This solar air conditioning case study is just a good example of how to calculate and size an indoor solar air-conditioning system that uses an absorption chiller unit operating with water and lithium bromide (Li Br).
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