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HEATING AND REFRIGERATION BY SOLAR ENERGY

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HEATING AND REFRIGERATION BY SOLAR ENERGY

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INTRODUCTION

The considerations put forward in the design of water heating systems can be extended to solar heating and cooling systems. The basic building elements of the most common solar water heaters are the flat plate manifold and the storage tank.

The manifolds are connected to cover a load, (auxiliary power is generally available), and means for circulating water and system control must be included; a practical outline of an example of a natural circulation system; in this device the reservoir is located above the collector and the water circulates by natural convection as long as the solar energy in the collector provides sufficient energy to the water that rises through it, thus establishing a gradient of densities that causes the movement of the fluid by natural convection. The auxiliary energy is applied at the top of the tank and is intended to keep the hot water in this area of ​​the tank at a minimum temperature level, necessary and sufficient to cover the loads and maintain the circulation.

In systems with forced-circulation water, where it is not necessary to place the reservoir above the manifold, although a pump is usually necessary, which is usually controlled by differential control, which actuates and starts it when the temperature 
detected by a sensor placed at the outlet of the manifold is several degrees above the water temperature at the bottom of the reservoir. 
A check valve is also required to avoid reverse circulation during periods of inactivity of the manifold, including night, and corresponding nighttime thermal losses. In these schemes it is shown that the auxiliary energy is supplied to the water between the outlet of the storage tank and the load.

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COLLECTORS AND STORAGE TANKS 
The type of plate collectors most commonly used, in which it is observed that the collecting tubes through which the water circulates to be heated, are arranged in parallel and have diameters between 1.2 cm and 1.5 cm with a separation between 12 and 15 cm and are welded or sausages both to the manifold plate and to the manifold manifolds, having a diameter of approximately 2.5 cm.
The collector plates are generally made of copper, although there are systems that use galvanized iron collecting plates; the absorption plates are mounted in a metal or cement box with an insulation of 5 to 10 cm in thickness on the back side of the plate and with one or two glass covers, so that for the air chamber a separation between them of the order of 2.5 cm
The dimensions of a collector are normally 1.2 x 1.2 m2, and it is possible to use groups of collectors mounted in series, in parallel or in other arrangements. 
Other types of tubes may be used to transfer the energy captured in the manifold plate to the flowing fluid, such as a single serpentine tube instead of the parallel tubes, thereby eliminating the end manifolds, or a set formed by a flat plate and another corrugated one united by electrical resistance welding, in such a way that through the undulations between plates circulates the water Storage tanks have to be insulated thermally; a mineral wool insulation can usually be used on the sides, at the top, and at the bottom, about 20 cm thick; it is also necessary to thermally insulate the pipes from the collector to the tank, so they have to be designed and calculated very well, to minimize pressure drop and pressure drop; pipelines with a diameter of 2.5 cm or more are used in dwelling units with lengths as short as possible.
It is necessary that the stratification can be maintained in the storage tanks within limits, so that both their location and position, and the design of the connections of the tanks is very important.

SOLAR HEATING

The heat necessary for the conditioning of buildings can be supplied by solar energy techniques with systems that, conceptually, are only larger versions than those used in water heating.
The most commonly used fluids for heat transfer are water and air.
In temperate climates, one has to have a conventional auxiliary energy source design problem is reduced to deciding the optimal combination between solar and auxiliary energy.

The so-called solar houses that have been constructed are buildings with large windows facing
the Ecuador, designed to admit solar radiation when the sun is low during the
winter.
The thermal gains that can be achieved with properly oriented windows are significant,
although in cold climates it is very important to control thermal losses during periods of low solar radiation, especially at night and cloudy weather, so you can achieve adequate profits.

SOLAR HEATING SYSTEMS.- The main components of solar
heating systems are:

a) The collector
b) The storage system
c) The conventional auxiliary power source
For these systems it is useful to consider four basic operating modes
depending on the conditions existing at a given time:
a) If solar energy is available and not heat is required in the building, the energy gain from the
collector is added to the storage system.
b) If solar energy is available and hot in the building, the energy gain is used to cover other building needs.
c) If there is no solar energy available, and it is necessary to apply heat in the building and the storage unit has a certain amount of energy stored, it uses that stored energy to cover the needs of the building.
d) If there is no solar energy available, and

SOLAR HEATING

The heat necessary for the conditioning of buildings can be supplied by solar energy techniques with systems that, conceptually, are only larger versions than those used in water heating.
The most commonly used fluids for heat transfer are water and air.
In temperate climates, you have to have a conventional auxiliary energy source. The design problem comes down to deciding the optimal combination of solar energy and energy

assistant.
The so-called solar houses that have been constructed are buildings with large windows facing
the Ecuador, designed to admit solar radiation when the sun is low during the
winter.
The thermal gains that can be achieved with properly oriented windows are significant,
although in cold climates it is very important to control thermal losses during
periods of low solar radiation, especially during night and cloudy weather, in order to achieve
adequate profits.
SOLAR HEATING SYSTEMS.- The main components of solar
heating systems are:
a) The collector
b) The storage system
c) The conventional auxiliary power source
For these systems it is useful to consider four basic operating modes,
depending on the conditions existing at any one time:
a) If solar energy is available and there is no need for heat in the building, the energy gain from the
collector is added to the storage system.
b) If solar energy is available and hot in the building, the energy gain is used to cover
other building needs.
c) If there is no solar energy available, and it is necessary to apply heat in the building and the storage unit
has a certain amount of stored energy, it uses that stored energy to cover the needs
of the building.
d) If there is no solar energy available, and

SOLAR HEATING

The heat necessary for the conditioning of buildings can be supplied by solar energy techniques with systems that, conceptually, are only larger versions than those used in water heating.
The most commonly used fluids for heat transfer are water and air.
In temperate climates, you have to have a conventional auxiliary energy source. The design problem comes down to deciding the optimal combination of solar energy and energy

assistant.
The so-called solar houses that have been constructed are buildings with large windows facing
the Ecuador, designed to admit solar radiation when the sun is low during the
winter.
The thermal gains that can be achieved with properly oriented windows are significant,
although in cold climates it is very important to control thermal losses during
periods of low solar radiation, especially during night and cloudy weather, in order to achieve
adequate profits.
SOLAR HEATING SYSTEMS.- The main components of solar
heating systems are:
a) The collector
b) The storage system
c) The conventional auxiliary power source
For these systems it is useful to consider four basic operating modes,
depending on the conditions existing at any one time:
a) If solar energy is available and there is no need for heat in the building, the energy gain from the
collector is added to the storage system.
b) If solar energy is available and hot in the building, the energy gain is used to cover
other building needs.
c) If there is no solar energy available, and it is necessary to apply heat in the building and the storage unit
has a certain amount of stored energy, it uses that stored energy to cover the needs
of the building.
d) If there is no solar energy available, and there is a need for heat in the building, and the storage unit is depleted, then the conventional auxiliary power needs to be used to cover the needs of the building.

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 It is necessary to take into account a fifth situation according to which the storage unit can be totally heated, without loads to cover, and with the collector in situation of gaining energy; in these circumstances there is no way to use or store the collected energy and it can not be used, so it has to be wasted.

 SOLAR REFRIGERATION BY ABSORPTION

Techniques such
as those proposed below may be used for the operation of absorption chillers by solar energy. A
) Use continuous chillers, similar in construction and operation to conventional gas or condensable fluids units; the power is supplied to the generator from the solar collector system, provided conditions within the building indicate the need for cooling.
b) Use intermittent chillers, similar to those used in food refrigeration, marketed for years in rural areas, before compression refrigeration and electrification were extended.

No intermittent chillers are used for air conditioning, nor have there been any
great studies that advise its possible application to the air conditioning by solar energy.
It is possible to adapt flat plate manifolds to operate with absorption refrigeration cycles,
the influence of temperature limits on the operation of flat plate manifolds, makes it possible to consider only commercial machines with lithium-water bromide systems, Li -Br-H2O, which require cooling water to cool the absorber and the condenser, so that its use may require the use of a cooling tower.

The use of ammonia-water coolers in the way they are currently marketed
is difficult because of the high temperatures required by the generator, which would require parabolic cylindrical collectors

 Using chillers designed to run on solar energy involves generators with lower operating temperatures, which means better levels of energy input to the generator from the collector and better operation within a given temperature range.

If cooling demands, rather than heating loads, set the size of the collecting surface, it is advantageous to design coolers with a higher COP than usual; for example, double-effect evaporators can be used to reduce energy input requirements, which means that both conditions and restrictions for operation with solar energy may lead to different refrigerant designs than those used to operate fuels in conventional systems.

A calculation of costs indicates that the operation of these systems is competitive with the classic compression that works electrically.
Studies of solar energy in a series of ammonia-water coolers using
flat plate collectors without storage have shown that the temperature range
for the water supply to the generator has to be between 60 ° C and 90 ° C, not specifying the water temperature of the condenser. The concentrations of ammonia in the absorber and in the generator range from 58% to 39% depending on the power supply.
Intermittent absorption cooling is an alternative to continuous systems;
the work done on these cycles has been directed mainly at refrigeration for food preservation, rather than air conditioning systems.
In the air conditioning cycles, the distillation of the refrigerant from the absorber is performed during the regeneration step, condensing and storing the refrigerant; during the cycle cooling step, the refrigerant evaporates and is reabsorbed.
Storage in the form of absorbent and refrigerant offers variants of this cycle using pairs of evaporators and condensers, as well as other devices, which provides essentially continuous cooling capacity and improved COP.
The refrigerant-absorbent systems used in intermittent cycles are mixtures NH3-H2O and
NH3-NaSCN; in the latter, the absorbent is a solution of (NaSCN) in NH3, the NH3 acting
as a refrigerant, a system which presents good thermodynamic properties for cycles for
the manufacture of ice, by means of an intermittent NH3-H2O cooler, using for regeneration a parabolic cylindrical collector.

Experiments have also been developed on intermittent machines operating with NH3-
H2O in which the energy input is made by flat plate collectors, and in which the
absorber and the generator are separated. The generator forms part of the flat plate manifold; the
fluid flowing through the tubes is a refrigerant-absorbent solution through a combination of thermosyphon and bubble pump.

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SYSTEMS OF COLLECTION AND STORAGE OF HEAT IN WALLS

An interesting feature of solar energy storage used in buildings
is the collector-storage combination, in a single structural part of it, such as
a south-facing wall. The walls are vertical, whereas the angle of incidence qs of
the solar radiation on them, is high in winter and low in summer, reason why they are systems adapted to operate in winter.
An experimental structure of this type consists of a series of small cubicles along
the south-facing façade, composed of a double crystal and a wall behind the crystals
made of a material suitable for storing heat. For insulation can be used
umbrellas and for air circulation fans that control the losses and increase the speed of heat transfer from the storage wall to the room.
Cover with two crystals; at a distance of 10 to 20 cm from the roof is a concrete wall about 20 cm thick, painted black, which serves both as a radiation absorber and a heat storage medium. In the upper and lower part of the concrete wall there are openings or grids, so that the air circulates by natural convection, through the space between the glass and the concrete wall, not needing any external element.

SYSTEMS WITH HEAT PUMP AND COLLECTOR RADIATOR

In order to supply heating or cooling to buildings, there are systems that use
uncovered collectors such as day collectors and night
heaters, cold and hot water storage tanks , and heat pumps, which ensure adequate temperature differences
between them.
For single-story buildings with south-facing roofs, in places where
solar radiation is high, with little rain, warm summers, mild winters, and small
wind speeds , these can be covered with copper plates with built-in tubes , painted a dark
color, which act as collectors of solar energy to heat water, and as radiators to
radiate heat to the night sky.

A vertical water tank, divided in its mid-point by a thermal deflector, provides storage of both hot water in the upper section and cold water in the lower section; in addition a heat pump can be provided for the transfer of heat from the cold section of the tank to the hot one; the condenser coil must be in the upper section.

 The operating system, which can be:
- Heating only
- Cooling only
- Heating and cooling
a) For heating only operation, solar energy is collected when possible, and circulated water heated to the bottom of the tank; when the room thermostat requests heat, the hot water stored in the top of the tank is conveyed to the heat radiation panel.

The heat pump operates in the sense of pumping heat from the bottom to the top of the tank in order to raise the temperature at the top of the tank to levels at which the heat requirements of the building.

b) For cooling-only operation, the water in the upper tank is cooled by the radiator-collector night-time radiation; the cooling water is withdrawn from the bottom of the tank to cool the building, and when required, the heat pump lowers the water temperature in the lower section.

(c) For operation as heating and cooling, in spring and autumn, solar heated water is stored at the top of the tank, and water cooled by radiation at the bottom of the tank; both heating and cooling of the building are obtained from the corresponding upper or lower tank section.
The heat pump operates in the direction of raising the temperature in the upper section, or
lowering the temperature in the lower section, according to the needs of the building. With the
systems used in the cooling of the building by means of radiant panels, it is not possible to obtain a good humidification, since these systems extract the heat of the room air, at temperatures well above the temperatures to which the evaporators of the normal air conditioners, so their use is restricted to be used only in dry climates, if the corresponding dehumidifiers are not available.

The COP of the solar collectors is higher at low temperatures, with the COP of the
heat pumps being higher when the evaporator temperature is higher, so
the use of the solar collectors as thermal sources for the pumps has been considered of heat.
One scheme of such a system is shown in Fig. VI.16, the storage
on the evaporator side of the single-stage heat pump being shown. The
storage of heat on the side of the condenser can also be arranged ; in this case, the capacity
of the heat pump must be adapted to the maximum available energy in the manifold, rather than to the maximum
heating loads of the house, other variants being possible.
The COP and the cost of the manifold and the heat pump are improved when the two elements work
together instead of separately; when buying the two components, it must be taken into
account that this can represent a greater investment, as well as the disadvantage of having to use
electrical energy, which could cause an unfavorable distribution of loads in the
generating plants .

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