Patent Application
20150040888
1. Field of the Invention
This invention relates to solar heating of water for industrial and residential applications, and more specifically it relates to evacuated tube collector (ETC) solar water heaters, which use the heat pipe for effective and high temperature water heating and describes a new method to integrate phase change material (PCM) inside the inner space of ETC, in order to store heat and transfer effectively and use it with delay at needed time through heat pipe in contact with the PCM.
2. Description of the Related Art
There are two components required for a functional solar heater: the collector and the storage unit. The collectors are usually in the form of a flat plate or an evacuated tube solar collector (ETSC or shorter ETC). The storage unit is required because at certain times only a very small amount of solar radiation or no radiation will be received. Developing efficient solar energy storage and accumulation devices is an ongoing research by many scientists.
Among the available techniques suitable for storing thermal energy and for controlling temperature of systems subjected to periodic heating, the use of solid-liquid phase change materials has attracted considerable attention. Phase change materials (PCM) can store 5-14 times more heat per unit volume than sensible storage materials while the absorption and the release of that heat happen at a nearly constant temperature. Therefore, PCMs are suitable for energy storage and temperature control systems.
When a temperature peak occurs and temperature reaches a certain value known as melting point Tm, PCM melts and absorbs the excessive energy (known as latent heat) by undergoing a phase transition and releasing the absorbed energy later or when the demand peak has passed. Before a solid can melt, it must absorb sufficient energy to overcome the binding forces which maintain the solid structure. This energy for complete melting is referred to as the latent heat of the material and represents the difference in thermal energy levels between liquid and solid states. Conversely, solidification of a liquid requires the removal of this latent heat with the consequent structuring of atoms into a more stable lattice.
There are a large variety of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. Examples of phase change materials include water, salt hydrates, certain hydrocarbons, metal alloys and paraffin. They have large spectrum of latent heats and melting temperatures. Therefore, the PCM melting and solidifying temperature range can be easily matched with the system's operating temperature for the phase-change process to be effective. Other advantages of these PCMs include: physical properties, chemical properties (no toxicity and no fire hazard), and low cost and availability.
There have been many investigations on the utilization of PCMs inside storage tanks in solar applications. One of the challenges in the application of phase change materials is increasing their thermal conductivity while maintaining their high latent capacity. Recently, the combination of carbon nanotube arrays and PCM has been demonstrated to be very attractive for enhancing the thermal conductivity of the PCMs. Carbon nanotubes (CNTs) with extremely high thermal conductivity have shown tremendous potential for heat transfer applications. Compared with the other heat transfer promoters, CNTs yield better stability in an organic matrix. Several researches have been reported wherein CNTs are embedded in the base fluids and phase change materials to increase their thermal conductivities.
Although PCMs are good candidates for storage applications, not all PCMs are ideal for our novel idea. The PCM to be used in the evacuated tube solar collectors should have a high melting temperature, between 100° C. and 180° C. PCMs with high melting temperature usually have low thermal conductivity, high cost, subcooling, and poor stability during the temperature cycling; the number of suitable phase change materials which are suggested in the literature is very limited. TABLE 1 gives several suitable PCMs for our applications. All of these materials are safe, non-expensive and nontoxic with high melting temperature and latent heat.
The claimed invention relates to a system that combines the collection and the storage of thermal energy both in a single unit, and namely inside the evacuated tube of solar collector. Previously PCM has only been used inside flat panel collectors, and to our knowledge there has been no integration of PCM inside evacuated tube. The integrated collector is, in general, simpler than conventional domestic water heating system with PCM inside storage tank and has many advantages, including weight and cost savings.
Among the above PCMs, paraffin and erythritol have been chosen as preferred embodiments that are safe, non-toxic, non-expensive and readily available. The thermal properties of paraffin and erythritol make them excellent phase change materials.
An objective of the invention is to design and develop an effective method to store heat inside of an evacuated tube solar collector (ETC) for delayed release. An embodiment of the invention is directed to a previously developed evacuated tube collector, which comprises of two concentric glass tubes: the first inner glass tube is sealed on one end and fused to the second outer glass tube on the other end via a flared edge, the second tube is sealed at the free end, vacuum is created between the first tube and the second tube, the outside of the inner tube is coated with highly light selective “black” layer, and a heat pipe supported by a metallic fin is inserted inside the inner tube space, connecting it to the heat exchanger manifold. The claimed invention involves filling the void space of the first inner tube, where the heat pipe is located with phase change material in such a way that it is thermally connected with the heat pipe via fins or other media. Moreover connection of PCM to heat pipe can be switched either “ON” to release the latent heat stored in PCM into heat pipe and thus into manifold or “OFF” to keep heat accumulated and stored for later transfer. Alternatively, release and storage of the energy in PCM may happen by controlling the flow of water through the exchanger manifold. The heat is then accumulated effectively from solar radiation in a “stagnated mode”, when water flow is OFF, and released later, after sunset by switching the water flow ON. In case of a closed loop system an anti-freeze fluid may be used instead of water as heat transfer fluid.
Solar water heater (SWH) is a cost-effective way to generate hot water for commercial or residential applications. Solar water heating systems include storage tanks and solar collectors. Two types of solar collectors most frequently used are: flat plate collectors and evacuated tube collectors.
Experiments and simple calculations have show that the stored energy inside the tube can reach about 100 to 600 kJ/kg. This energy is enough to keep heat pipe generating energy transferred to the manifold with the outer water circuit for about 2-3 hours depending on the type of heat pipe and the type of PCM. If the heat pipe is not inserted in the evacuated tube then the energy stored in the PCM can be stored for many hours and experiments show that it can be stored without significant change in temperature for 10 hours. This means that the system can be switched ON anytime for operation depending on the demand. The heat pipe can start operating when the customer needs hot water. That operation requires a special switch, which starts the operation of a heat pipe, directly from PCM inside the inner tube. The system is turned ON by mechanically placing the heat pipe in contact with heat transfer fins inside the tube, as shown in
The phase transition of PCM between solid and liquid causes significant change in the volume ΔV of the material. The volume change can cause damage to glass ETC tubes. Cracking of the glass tubes can be avoided by partially filling the void inside the ETC tube. The PCM volume will change and return to original volume at room temperature.
In one embodiment, the operation of a solar water heater system is improved by the addition of metallic reflector (e.g., corrugated galvanized steel panel) behind the ETCs. The additional reflector increases the incident insolation on the collector array and provides heating along most of the entire diameter, as compared to only the top half of collector array being insolated without the back reflector. With more uniform radial heating of the collector, the heat pipe operates more efficiency when placed in the middle of the collector, as shown in
In order to evaluate the performance of phase change material inside evacuated tube collector, field test on 0.5 meter collectors were carried out.
The operation of two evacuated solar tube solar collector systems with and without PCM filled tubes were also compared. Each solar system had 10 evacuated tube collectors attached a heat exchange manifold and a pump circulated 40 liters of water through the systems. A flow rate of 0.2 GPM was maintained during the warm up and cool down of the system. For each system we recorded the temperature of the water in the tanks, the temperature of the manifold and temperature inside the ETC. In normal operation the systems were operated with constant water flow between the collector and tank. Both solar collectors were allowed to operate under the sun for several hours. The systems were then covered at the same time and water kept flowing with constant rate. Temperatures recorded during warm and cool down of system and are shown in
In
We observe the temperature inside evacuated tubes to be significantly higher compared to operation in normal mode, since no heat transfer to water is possible. We can assume that phase change of paraffin has occurred for the majority of the material since the temperature inside the tubes with paraffin has exceeded 73° C. We observe the fast cool down of standard evacuated tube (∘) and the delayed cool down of PCM tubes (). While systems are stagnated, the majority of absorbed energy is accumulated in PCM materials. Once the water flow is started, the energy can be transferred to water through the heat pipe and the manifold assembly. Therefore, the PCM filled tubes enable operation of the system in the dark. The paraffin filled ETC tube cool down curve shows a plateau around 73-75 C during cool down. The heat exchanger manifold of typical solar collector begins cooling down rapidly (□) in
Erythritol has higher of heat of fusion (known also as latent heat) of 340 KJ/kg and also increased melting point. The phase transition point is 123° C. We compared the operation of a solar collector filled partially with erythritol and a typical solar collector with empty ETC. The PCM filled collector was consisted of 10 filled tubes with erythritol inside mylar baggies and heat transfer oil. In
Two systems (typical ETC system and PCM integrated system) were operated daily under similar conditions. The solar water heaters were operated in two different modes, first in normal operation the water is circulated between the manifold of the collector at a constant flow rate and the water tank. Second, in stagnation mode, no water flow was initiated through the manifold for an extended time and the pump was not started until PCM in both systems was melted and collectors have been covered completely from sun. The temperatures of water in the storage tank of a typical solar water system and a PCM integrated systems are shown in
The storage tank temperature of the three systems is shown in
Peak solar energy absorption of an evacuated tube collector can be calculated by using the following assumptions: the absorber area of a typical 1.8 meter long collector tube is 2540 cm2; without reflective panels on the underside of the collector, the effective absorber area is reduced by 50%; solar irradiance is 100 mW/cm2; collector solar absorption is 92%. This results in a peak solar power absorption of 117 W per tube, corresponding to 117 W·h of energy every hour under the AM1.5 solar irradiance.
If the inside tube of the same 1.8 meter long evacuated tube collector was filled with erythritol, which has latent heat of 330 J/g (note that 1 Joule is equal to 1 W·sec) and density of 1.54 g/cm3, that would amount to approximately 3.86 kg of erythritol in the inner volume of 2660 cm3. This translates to stored energy of 1270 kJ. Estimating 66% efficiency of the latent heating with erythritol as compared to solar heating, the stored latent heat would provide the same amount of energy as 2 hours of peak solar power output. That is significant amount of stored heat available even at night.
ETC array used with reflectors for solar radiation concentration or parabolic trough collectors operate at higher temperature, as compared to the conventional solar water heater systems. Typical cermet solar selective coating of ETCs is limited in application to 400° C. due to the delamination of the selective coating from the substrate. For high temperature applications, such as ETCs with reflectors or parabolic troughs, carbon nanotube sheets can be used as the solar selective coating that are thermally stable above 600° C., which is the limiting temperature for the glass structure of the collectors. With successful implementation of solar selective coating with thermal stability above 600° C. a different type of phase change material must be used for effective operation.
Molten salts and salt eutectic systems are ideal for high temperature storage applications. Molten salt systems have a wide range of tunable temperatures from 250° C. to 1680° C. with latent heat from 68 to 1041 J/g. Significant disadvantage of these systems is their low thermal conductivity. In order to improve the thermal conductivity of PCM incorporated into a high temperature ETC or parabolic trough, carbon nanotubes are incorporated into the PCM mixture, as shown in
The flow rate of heat transfer fluid through the manifold of SWH system affects the performance of system in normal operation during the day and also during on-demand operation.
While particular embodiments of the present disclosure have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the disclosure. It is therefore intended to cover in the appended claims all such changes and modifications that are with the scope of this disclosure.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/863,797 filed Aug. 8, 2013, which is incorporated herein by reference in its entirety as if fully set forth herein.
This invention was made with government support under DOE Phase I STTR Grant No. 87938T12-I awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61863797 | Aug 2013 | US |
