GB2509795A

GB2509795A - A solar heat collecting arrangement which includes at least one thermal battery within a tube heated by solar radiation

Info

Publication number: GB2509795A
Application number: GB1309196.2A
Authority: GB
Inventors: Deju Denton St Leo Nation
Original assignee: University of Leeds
Current assignee: University of Leeds
Priority date: 2013-01-09
Filing date: 2013-05-22
Publication date: 2014-07-16
Anticipated expiration: 2033-05-22
Also published as: GB201300350D0; GB2509795B; GB201309196D0

Abstract

A solar heat collecting arrangement includes at least one thermal battery 17 within a tube heated by solar radiation. The batteries are thermal-electric batteries such as sodium sulphur, sodium nickel, ZEBRA, etc, and the tube is annular with the space bounded by an inner tube wall 15 housing the batteries and the annular space 12 between the inner wall and an outer wall 14 carrying a heat transfer fluid which provides for heating or cooling of the thermal batteries; the tube outer being heated by solar radiation. The annular tube can be enclosed by a transparent tube 11, which may be borosilicate glass, and a space between the transparent tube and outer tube may be evacuated. The heat transfer fluid may be synthetic oil, molten salt, steam/water, etc. The inner tube may also include a ceramic tube 18 to provide insulation between the tube wall and the battery. The arrangement allows electrical energy to be stored during sunlight hours within the battery, which can be released during periods when solar radiation is not available.

Description

Background the Invention

Concentrating solar power (CSP) power plants are the largest commercial producers of electricity from solar energy. There are four major types of CSP plants; parabolic trough collector, solar tower, linear Fresnel and solar dish power plants. Of these four, parabolic trough collector plants have the most proven track record along with a long and successful history of deployment which goes as far back as the 1980's. Parabolic trough plants consist of large fields of parabolic trough reflectors (mirrors) that concentrate solar radiation onto a central receiver tube or heat collecting element (HCE) in the focal axis of the reflector. The receiver tube heats up and heat energy is transferred through the receiver tube walls to a heat transfer fluid flowing inside.

It is agreed by experts in the field that with the projected future increase of renewable power sources connected to the grid, there will be an increased need for these sources to be able to store energy due to the intermittent nature of renewable power sources, namely the sun.

Parabolic trough power plants traditionally store power in thermal form using hot and cold molten salt tanks, and this is a proven storage method. However, since the energy in stored in thermal form, before it can reach the grid it has to be converted to electrical energy by the plants power block. This time delay lessens the ability of the power plant to respond to ramping of demand from the grid, and therefore places great demand or electrical stress on the plants ramping generators.

The ES-HCE (figure 1) is a special hybrid receiver that could provide a solution to this problem. It stores solar energy in electrical form in high energy density thermal batteries (17) placed inside the receiver (HCE) itself. These battery cells are electrical connected and controlled in series and/or parallel and may be sodium sulphur (NaS), Zebra, sodium nickel (Na-NiCI2) or any other suitable thermal battery. They are called "thermal" because they must be heated up to a certain operating temperature before they can be charged or discharged. These cells are currently being used to store energy for wind farm applications and grid support because they can store 4-5 times more energy than a traditional lead acid battery in the same volume. In the ES-HCE, these thermal cells are "cylindrical", like regular flashlight battery cells, possessing a positive (+) and negative (-) terminal for electrical connection.

They are heated up in the ES-HCE by heat passing to them through a battery tube wall (15) from a heat transfer fluid (HTF) flowing in the system. The heat transfer fluid (HIF) which may be synthetic oil, molten salt, steam/water or other appropriate fluid is heated up by solar heat absorbed through the receiver tube walls(14), which have a specially selective coating (27). Heat flow to and from the thermal cells is regulated by controlling the mass flow rate of the HTF and/or defocusing (moving out of line) the ES-HCE from the solar radiation using the tracking mechanism of the FTC. The ES-HCE also utilizes a borosilicate or similar transparent tube (11) with anti-reflective coating to cover the receiver tube (14) and prevent or lower convective heat transfer to the ambient.

Charging and discharging the thermal batteries Once the thermal cells inside the receiver have reached their operating temperature by heat transferred to them from the heat transfer fluid (early-mid morning), a portion of the electricity produced by the grid would be used for charging these thermal cells throughout the daytime.

During the evening or nighttime hours, this energy is discharged back to the grid. Charging and discharging is done by the use of suitable high efficiency (>90%) power electronic converters designed for this purpose.

Maior benefits the invention could provide The main benefits that this invention could provide are: * Energy is already in electrical form, therefore no thermal-electric conversion delay * Lower conversion losses in (Power electronics losses lower than heat exchange loss) * Allows power plant to respond virtually instantaneously to demands for stored energy.

* Better power quality of the power plant, better ramping response and voltage/frequency control * Battery discharge heat during no-sun hours helps to provide freeze protection in solar

fields with molten salt HTF

Description of the ES-HCE invention

Figure 1 shows a cross section of the ES-HCE. It can be seen that the ES-HCE is essentially a hybrid solar receiver, with the thermal batteries (17) placed on the inside and heat transfer fluid (HTF) flowing in the annular space (12) between the absorber (14) and battery tubes (15). Tubes

The NaS cells are placed inside a steel or aluminium "battery tube" (15) for protection, while allowing effective heat transfer to the cells. It can also be seen from figure 1 that a low cost ceramic tube (18) is placed between the NaS cells (17) and the battery tube (15), serving to electrically insulate the NaS cells from the battery tube, so as to prevent short circuiting. The battery tube (15) and the receiver tube (14) are spaced from each other by a special steel bush (3) which slides between them and has flow slots (25) for the heat transfer fluid to pass through. The space (37) between the transparent cover (11) and the receiver tube (14) is evacuated of air, with a steel bellows mating this cover to the steel receiver tube via a graded glass-metal seal (10) or other appropriate hermetic sealing device. The transparent cover (11) for passing solar radiation (20) may be totally circular (31) or it may have a parabolic glass segment (30) built into it for reflecting visible solar spectrum onto a photovoltaic cell strip placed directly below the receiver under the normal shadowed area.

The reflection of the visible spectrum onto the concentrating PV cells is achieved by coating the parabolic segment with a "cold mirror" coating (32). This cold mirror coating (32) passes the infrared and ultraviolet portions of the solar spectrum through the transparent parabolic segment for heating up the receiver tube (14), but reflects the visible part of the solar radiation downwards to a photovoltaic surface placed directly beneath and in line with the ES-HCE receiver.

Tube termination and inter-connection The battery tube channel (5) which contains the cells is closed by a special cap (1) which has a hexagon shoulder (23) and a cylindrical bung (24) which may be threaded for screwing in, or interference fitted to ensure a leak proof seal. The cap (1) also has a cone shape at the other end for providing a more streamlined HTF flow characteristic in the flow channel.

The steel bush sits inside an outer steel connector cap (4) with the bush face (26) mating with the connector cap face (22). There is a special set of battery wire holes (8) which must be aligned before the surfaces (34) are welded. It is important that the weld is leak free so as to prevent heat transfer fluid getting into the battery channel (5). The connector cap (4) is spot welded at certain points (35) to the steel bush (3). The connector cap surfaces (36) may be adapted in any way required (machined or welded connection) to mate other a given ES-HCE tubes to in series with another. A special compression spring or spring washer (19) serves to ensure that the cells are in firm electrical contact with each other.

Claims

Claims This invention claims: 1. Application to any solar heat collecting element (HCE) or receiver comprised of an inner tube containing thermal-electric batteries such (eg. Sodium Sulphur and Zebra) and an outer tube that is heated by solar radiation, with the annular space between the inner and outer tubes carrying a heat transfer fluid for heating up or cooling the thermal batteries.
2. That the heat transfer fluid used may be thermal mineral/synthetic oil, molten salt, ionic fluid, water/steam or any flowing heat transfer media appropriate to the temperature of operation at the given time.
3. Sole manufacturing rights for use of "this receiver" and this receiver concept" in any commercial solar thermal power plant, system or application. (Particularly in Parabolic Trough Collector (PlC), Linear Fresnel, Solar Dish and Power Tower solar power plants).
4. Applicability for use in use in both residential and commercial, small or large scale solar thermal power systems.Amendments to the claims have been filed as follows Claims This invention applies to: 1. A solar heat collecting element comprising an inner tube, an outer tube surrounding the inner tube and defining an annular space between the tubes through which in use, heat transfer fluid can flow, the inner tube enclosing a plurality of thermal-electric batteries and the heat transfer fluid within the annular space providing heating or cooling of the thermal batteries.2. A solar heat collecting element as outlined in claim 1, the heat transfer fluid (HTF) comprising at least one of a mineral oil, synthetic oil, molten salt, water/steam mixture, or an ionic fluid. C?) C?)