Explanation of major technologies
Evacuated tube solar panel
With advances in technology, solar water heating systems have become more and more efficient. One of the new high efficient technologies is the evacuated tube solar panel that offers efficiencies of well over 90%. This means that more than 90% of the sun's energy landing on the evacuated tube’s surface is converted into heat which is used to heat water.
Evacuated tube solar thermal collectors incorporate a number of technologies to greatly increase their efficiency (over the traditional flat panel collector) and maximum operating temperature:
- The evacuated tube (absorber) is insulated from the air by a vacuum. This greatly decreases convective losses to the wind
- Heat is transferred from the absorber to the working fluid by means of a heat pipe. A heat pipe is filled with a partial pressure of water, such that it boils at 86°F. When the water evaporates, it convects up to a heat exchanger where it condenses, transferring its energy to the working fluid. The effective thermal conductance is roughly 10,000 times that of a solid copper bar. The heat pipe also has a diode function, in that if it is not hot enough to evaporate the water, no convection will occur
- Evacuated tube systems are capable of limiting the maximum working temperature, where as Flat panel systems have no internal method of limiting heat buildup which can cause system failure
- Evacuated heat pipe systems are lightweight, modular, easy to install and require minimal maintenance. Flat panel systems, on the other hand, are difficult to install and maintain, and must be completely replaced should one part of the system stop working.
The result of these efficiencies is that the number of evacuated tube panels required is greatly reduced compared to the traditional flat panel design.
Absorption Principle
Absorption chiller uses a solution of lithium bromide and water (under a vacuum) as the working fluid. Water is the refrigerant and lithium bromide, a nontoxic salt, is the absorbent. Refrigerant, liberated by heat from the solution, produces a refrigerating effect in the evaporator when cooling water is circulated through the condenser and absorber.
The Cooling Cycle explained:
Generator
When the heat medium inlet temperature exceeds 154.4°F, the solution pump forces dilute lithium bromide solution into the generator. The solution boils vigorously under a vacuum causing it to release refrigerant vapor. After separation, refrigerant vapor flows to the condenser and concentrated solution is pre-cooled in the heat exchanger before flowing to the absorber.
Condenser
In the condenser, refrigerant vapor is condensed on the surface of the cooling coil and latent heat, removed by the cooling water, is rejected to a cooling tower. Refrigerant liquid accumulates in the condenser and then passes through an orifice into the evaporator.
Evaporator
In the evaporator, the refrigerant liquid is exposed to a substantially deeper vacuum than in the condenser due to the influence of the absorber. As refrigerant liquid flows over the surface of the evaporator coil it boils and removes heat, equivalent to the latent heat of the refrigerant, from the chilled water circuit. The recirculating chilled water is cooled to 44.6°F and the refrigerant vapor is attracted to the absorber.
Absorber
A deep vacuum in the absorber is maintained by the affinity of the concentrated solution from the generator with the refrigerant vapor formed in the evaporator. The refrigerant vapor is absorbed by the concentrated lithium bromide solution flowing across the surface of the absorber coil. Heat of condensation and dilution are removed by the cooling water and rejected to a cooling tower. The resulting dilute solution is preheated in a heat exchanger before returning to the generator where the cycle is repeated.
Note: the above description on the cooling cycle was reproduced from Yazaki Energy Systems, Inc. (http://www.yazakienergy.com)