Problem: Stirling cycle devices have typically been difficult to scale up to larger, more powerful units because the working fluid, often helium, must pass through various chambers, passages and materials in order to transfer heat energy to and from various surfaces. This balance of resistance vs. volume in a device that typically runs at a rapid cycle rate, makes it difficult to predict the performance of a similar device of a different size. In order to use energy most efficiently, these devices typically employ regenerators in the form of porous materials such as stainless steel spheres or stacks of stainless steel wire screens. While these regenerators can be sized to recover a significant amount of energy during each cycle, they add a great deal of dead volume to the system, thereby diluting any systemic pressure changes by allowing the system to cause a temperature change to the working fluid inside the regenerator’s dead volume, as well as other dead volumes such as in the heat exchanger passageways. The working fluid undergoing this temperature change within dead volumes is not in contact with any of the useful surfaces of the device, and therefore cannot provide useful work during that cycle.
Objective: Construction of a heat pump that eliminates the majority of its internal dead volume in order to efficiently use solar thermal energy, waste heat, geothermal energy or any other temperature difference to power heating, air conditioning, or power production.
The first goal is the construction of a regenerator/displacer cylinder that allows testing of the advantages and disadvantages of the elimination of the dead volumes. The cylinder assembly will have a heated head, a cooled head, and temperature and pressure measurement points. The interior will contain a stack of numerous nesting disks, containing geometries that are complimentary to each other, such that all working fluid is displaced from the interstitial spaces of the regenerator at the end of each cycle, thereby forcing all working fluid into spaces that can do useful work. Cycles will be manually timed and controlled. The pressure differences can then be measured at various phases of the cycle and adjustments can be made to optimize the variables.
The next goal will be to construct another cylinder that is similar to, but smaller than, the first, to test the actual compression effect of the driving cylinder on a driven cylinder under various circumstances to see whether the system can directly provide cooling.
The next goal will be to allow for the flexible timing of cycles in order to allows for a dwell period during any particular time in any cycle. An electronically driven timing/actuation system will need to be developed for this function. This will allow useful work to be accomplished as fully as practicable during a given phase of a cycle before proceeding to the next phase of the cycle.
A further goal is to reduce fluid frictional losses by cycling at a rate no greater than necessary to accomplish the presently assigned task. A flexible electronic timing system will also make this possible.
A further goal is to allow for multiple heat sources for the heating of the driving portion of the device, including solar thermal heating as well as electrical, gas or waste heat from a process, building or vehicle so the system may be used efficiently with any heat source.
A further goal is to allow waste heat from the warm side of both driving and driven cylinders to be reused for other purposes, such as water heating.
Finally, it is a goal to allow for the connection of this system’s data processing unit to the system control of an associated building, in order to better integrate all systems within the building efficiently.
Plan of Action:
Some preliminary modeling will be done using software appropriate for the task in order to size some of the components, but since performance of this type of device is typically difficult to model and predict, a series of experiments will be designed to study the actual performance of different materials of various sizes under the various temperatures and pressures and at different cycle rates. Two fiberglass cylinders that are lined with high temperature plastic sheet and capped with machined aluminum heads will suffice for the initial pressure vessels. The inner displacer/regenerator elements will be machined for the first tests, and will either be round grids of trapezoidal prisms or disks containing truncated conical bumps on one side and holes of the same shape on the other side. In any case, they will be constructed of metals of high thermal conductivity alternating with plastics of low thermal conductivity in order to store the heat energy efficiently without allowing a short circuit of axial heat conduction through the stack. Actuation of the cycles will be initially done manually, manipulating shafts that pass through seals in the cylinder to compress the displacer/regenerator stacks.
After the preliminary experiments provide data on which geometries and materials are most promising, a more complete system can be constructed, including an electronic control system to actuate the cycles at the most advantageous times, according to algorithms that use temperature and pressure data from sensors placed in various areas of the device.
The first experimental parts will be machined by a qualified machine shop. A thermodynamics consultant with a powerful piece of Computational Fluid Dynamics software will probably save some time (if affordable.) Experiments will be carried out locally (Stanford/Palo Alto/Silicon Valley area) and no travel is likely necessary.
Design of the initial experiments will accommodate simplicity and flexibility. Some initial caution in design can allow for adjustment of the experiments so that surprises and setbacks don’t necessarily call for the replacement of too much hardware.
The management goal is an affordable and practical initial experiment that results in data that justifies a more involved experiment, ideally to the extent that ongoing funding is attracted.
