Patent Application
20080022683
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments that are illustrated in the attached schematical drawings, in which:
The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.
The heat in the heat storage device 1 is preferably generated from renewable and intermittent energy sources. For instance, solar energy can be converted to electrical energy in photovoltaic cells, or wind energy, the most promising and most unpredictable renewable source of energy, is converted into mechanical energy in a wind turbine coupled to a generator 41.
The conversion of electricity to heat is done via resistors 43 distributed throughout the heat storage unit 1. These resistors need to be in good thermal contact to the surrounding heat storage medium 11 so that they can transfer the heating power to that medium. Possibly, the resistors could be simple steal ribbons buried between an appropriate solid refractory castable. The lower the heat resistance to the bulk of the heat storage medium, the lower is the tendency of the resistors to overheat. The thermal expansion coefficients of the resistor material and the heat storage medium need to be approximately matched or the design must accommodate the difference in thermal expansion. The supply voltage is a compromise between two contradicting requirements. On one hand, the resistors will overheat less if they are widely spread throughout the heat storage medium. Taking into account reliability, this favours a low supply voltage, namely many resistors connected electrically in parallel; on the other hand, the electrical power supply is a heat leakage path which cuts through the heat insulation, having in mind the relationship between electrical and thermal conductivity in metals (Wiedemann-Franz law), this would favour a high supply voltage, i.e. low currents.
The storage of the heat is either in sensible form in a solid refractory or in latent form (in the melting energy) of a salt or a metal. The heat storage material preferably has the following properties: high density, high heat conductivity, chemical stability and compatibility at the working temperatures. In addition, a high specific heat capacity over the relevant temperature range for sensible heat storage or a high specific melting energy and appropriate melting temperature for latent heat storage are required. Typical values are listed in the following table:
Heat is collected from the thermal storage unit with heat exchanger tubes or pipes distributed throughout the thermal storage unit and arranged to minimize the temperature drop to the heat storage material. Because in wind energy storage, the rate for charging (i.e. heating) is usually considerably higher than the rate for discharging, the arrangement of said heat collecting tubes is less critical than the distribution of the heat generating resistors.
State of the art large coal-fired steam power plants operate at conversion efficiencies of 41-45% from the chemical energy content of the coal to electricity. The upper working temperature of the Rankine cycle is today limited to ca. 650° C. because the aggressive steam under high pressure limits the lifetime of the steal tubes in the steam generator (the heat exchanger to the combustion chamber). In consequence, a steam turbine fed from a thermal storage unit hotter than 650° C. would need an additional controllable heat transfer between the main thermal storage unit and the steam generator. This complicates the design by introducing two more heat exchangers. On the other hand, such a design permits higher upper temperatures for the heat storage unit and thereby decreases the cost.
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Because the efficiency of the thermal energy storage technology is inherently restricted, its beneficial use is limited to very particular economic boundary conditions, namely a large difference between the value of electricity going into the unit and the value of electricity coming out of the unit. With the reduction in wind power equipment prices and the cost of fossil fuels and/or their combustion products this is the case for wind power. Wind is a free fuel and the value of wind power when there is too little load demand is essentially zero, and the value of wind power when there is demand is considerable indeed. Under these circumstances, a combination of electrothermal energy storage and combustion of (fossil) fuels provides for a cost efficient system for storing energy and an economical way of generating electricity.
In the following, a practical embodiment of the invention is given:
It is assumed that the magnesia refractory uses the temperature range between 400° C. and 800° C. The storage unit is “empty” at 400° C. and “fully charged” at 800° C. The upper temperature is defined by the upper working temperature of the heat exchanger tubes used to extract the heat. The minimum temperature is defined as the lowest reasonable working temperature of a state-of-the art Stirling engine. In consequence, the part of the heat contained below the lower temperature is not used.
State-of-the art Stirling engines are known to achieve 65-70% of the Carnot efficiency for conversion of heat to work. With the Carnot efficiency, this ηCarnot=1−TLow/THigh, this would mean at least 65%×73%=48% at a temperature of 800° C. (when the thermal storage unit fully charged) and 65%×57%=37% at 400° C. (when the thermal storage unit is discharged to the practical limit), assuming TLow 15° C. The weighted average would be 43%.
In order to store 12,000 kWh of thermal energy between 400 and 800° C., one needs 32.8 m3 or a cube of 3.2 meter side length. This corresponds to a gross effective thermal energy density (between 400° C. and 800° C.) of 370 Wh per litre and 122 Wh per kg. (Due to the heat insulation, see below, the net effective energy density is smaller. For the example system, a quick calculation gives a 55 cm thick heat insulation wall. This makes the three meter cube of the heat storage material a 4.4 meter cube in total, it adds as much as 44 m3 to the 33 m3 “active” material. Assuming realistic temperature dependent heat conductivity properties of this heat insulation system, the storage unit would lose 5.1 kW thermal power continuously (at 800° C.) through the heat insulation. This corresponds to a 1.0% self discharge per day. (The heat leaking through the electrical supply to the resistors is only ca. 540 W even at the low supply voltage assumed above.) Assuming a heat transfer coefficient of 1000 W/(m2K), which seems conservative e.g. for a steam generator or the working gas of a Stirling engine, and permitting 10° C. temperature difference from the tube to the fluid means that the heat exchanger needs a surface area of 25 m2, it could tentatively be realized with 63 pipes of 4 cm in diameter and 3.20 m length (connected partially in series and in parallel).
| Number | Date | Country | Kind |
|---|---|---|---|
| 04405156.3 | Mar 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH05/00142 | 3/10/2005 | WO | 00 | 9/15/2006 |
