Solar thermal collector system and method for flat roof constructions

Abstract

Disclosed herein is a solar thermal collector system that is particularly configured for installation on a flat roof of a building. In accordance with aspects of a particular embodiment of the invention, the solar thermal collector system includes a solar thermal collector module having a glazing sheet at a top, exterior surface, and an absorber sheet within the module positioned below and spaced apart from the glazing sheet. At least the absorber sheet is fluidly connected to a fluid handling system, and carries a working fluid that is heated in the module by the sun and transfers such heat to equipment connected to the module by the fluid handling system. The module is preferably provided a thermally actuated valve that allows the working fluid to also flow through the glazing sheet, which results in self-regulation of the temperature of the module below a critical design temperature.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This application is based upon and claims benefit of copending U.S. Provisional Patent Application Ser. No. 61 / 928,111 entitled “Systems and Methods for Solar Heating and Cooling of Buildings,” filed with the U.S. Patent and Trademark Office on Jan. 16, 2014 by the inventor herein, the specification of which is incorporated herein by reference in its entirety.FIELD OF THE INVENTION[0002]This invention relates to radiant energy management, and more particularly to a solar thermal collector system configured for use on buildings having a flat roof construction.BACKGROUND OF THE INVENTION[0003]Skylight systems have previously been provided that are capable of providing the majority of the lighting needs for various flat roof commercial buildings. In such systems, the skylight may convert the excess solar energy that is not needed for illumination into thermal energy that can be used for process hot water, space heating, and solar cooling. Sol...

AI Technical Summary

Benefits of technology

This patent is about a solar thermal collector system that can be installed on a flat roof of a building. It includes a module with a glazing sheet and an absorber sheet that is heated by the sun and uses a fluid handling system to transfer the heat to external equipment. The module has a valve that allows the fluid to flow through the glazing sheet, which helps to keep the module cool. The technical effect is that this system can efficiently collect and utilize solar energy on a building roof.

Problems solved by technology

Even at relatively high thermal efficiencies, this only provides about one fourth of the thermal energy needed to cool the entire building area below the skylights.

However, the cost of installation per unit of thermal energy generated by conventional thermal collectors is much higher than that generated by previously presented energy managing skylights, and the overall project economics can be significantly degraded.

This has the advantage of providing firm capacity during periods of low sunlight, but the expense of the gas backup degrades the project economics and the additional fossil fuel use works against one of the main product objectives of being a primary renewable energy source.

One problem with night sky radiant cooling is that the typical cooling heat fluxes are only about 1 / 10 those which can be achieved during solar collection of sunlight.

Also, the cooling effect is generally uncorrelated in time with the cooling loads.

Several techniques have been described for night sky radiant cooling, such as flushing the roof surface with water at night, and using relatively low efficiency solar collectors as radiators at night, but these techniques are not practical as yet.

The fundamental problem with using solar collectors as radiant cooling devices is that the design of the collector is intended to thermally isolate the fluid from the ambient air and the radiant sky environment.

These high stagnation temperatures then drive the need for even more expensive materials and components to ensure that the panel does not damage itself in full sunlight.

The drive for higher efficiency and the need to withstand high stagnation temperatures increases the cost per unit area of the solar collector.

The relatively high cost per unit area, plus the perceived need to generate the most energy in the winter, generally makes it imperative to orient the collector in an optimal position to capture the most possible sunlight to convert to thermal energy.

Orienting the panel at such a relatively high angle to the horizontal roof brings many problems and costs to the system installation.

On buildings with large flat roofs in relatively high wind areas, it is often not practical to even install large numbers of flat plate solar collectors due to the structural reinforcements that would be required to handle the wind loads.

Evacuated tube collectors may have lower but still significant wind loads.

The wind loads also drive significant structural requirements for the panel itself.

Furthermore, mounting the panel at an inclined position exposes the back of the panel to ambient air, resulting in the need for thick insulation to prevent significant loss from the back of the collector.

As the heat is conducted along the relatively thin absorber surface, there is a significant temperature drop between the absorbing surface and the working fluid.

This temperature drop results in thermal losses of between 12 and 18 percent, because the higher temperature of the absorber surface compared to the fluid temperature results in higher losses to the environment.

In addition, an efficient fin design requires creating a good thermal bond between the flat sheet and the fluid tube, which is a significant design challenge that drives up costs and creates failure points.

Further, nearly all current designs place the absorber surfaces directly under the glazing.

This causes a direct convective and radiative coupling between the two surfaces, accounting for the majority of the heat loss from the collector.

The glazing is typically made of low-iron glass, which has a high light transmissivity of about 90% but which is heavy, is a very poor insulator and so does not maintain more than a few degrees temperature difference across it.

In addition, many such coatings make use of toxic materials that require special handling, all adding considerable expense to the finished product.

There are several fundamental limitations which have thus far prevented the deployment of high efficiency, low cost, polymer (i.e., plastic) collectors.

The first problem is the low melting point of plastics that are sufficiently low in cost to be considered for use as collectors.

Secondly, extruded panels with discrete flow channels must be connected to a header or manifold.

The irregular shape of this welded joint makes the joint difficult to fabricate and prone to leakage with thermal cycles.

Further, all plastics have very low strength and stiffness relative to metals.

This makes it difficult for plastic solar panels to contain typical aqueous heat transfer fluid that in ordinary solar thermal systems can reach pressures of 150 PSI.

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