With the needs, goals and objectives identified, top level system requirements can be defined. These top level requirements will provide the basis for lower level requirements, and must be verifiable (e.g. by analysis, test, inspection, or some other method). They must also be upwardly traceable to the objectives, and flow down to lower level requirements. Requirement definition and analysis is generally an iterative process, but the top level requirements should document the primary system drivers that often come from the key stakeholders as "non-negotiable". Without this step in the process, there is no definitive measure to assess whether the system solution is acceptable.
An equally important step is drafting a system concept of operation. What are the nominal operations for the system? What are the key duty cycles and time frames over which the operations occur? What off-nominal operations are anticipated for maintenance, repair, etc.? By thinking through the specifics of these and other operational questions, the range of feasible solutions begins to come into focus. Bypassing this important step can lead to over-designed or under-designed solutions that become more costly to correct later in the development cycle.
A simple example of one top level requirement and a brief concept of operations summary:
- Requirement: The system shall be capable of removing 3000 W of thermal energy from the dewar continuously at 20 K. Rationale: Cooling capacity must be sufficient to overcome environmental heat leak and thermodynamic conditioning heat loads. Verification: by analysis and test.
- Concept of Operation: During nominal operations, the refrigeration system maintains tank pressure without venting. For tanker off-loads, roadable tankers fill the large stationary dewar while the refrigeration system conditions the hydrogen to maintain tank pressure without venting. When rapid depressurization is needed, the refrigeration system cooling flow is reversed to provide maximum cooling in the ullage for liquefaction. During fueling operations, the refrigeration system is in bypass mode. After fueling (post launch), the refrigeration system may be used to re-liquefy hydrogen vapor and depressurize the dewar.
System Trades and Analysis
The next step is to identify system design options that are within the feasible trade space, and then analyze critical performance and economic parameters. There are many possible trades to consider, but the primary ones from a thermal performance standpoint can be broadly grouped as passive, hybrid, active, or some combination thereof. Note that many options may fit into more than one of these categories depending on the implementation.
Passive techniques minimizing heat treat transfer into the system through structures, insulation, and penetrations are always important to explore first. These methods rely on good cryogenic design and material selection, and do not need any power input. Depending on the requirements and concept of operation timelines, an optimized passive thermal design may be sufficient to develop a no-loss system within the operational parameters of the application.
The hybrid category often relies on driving pressure differentials or power input for pumps, mixers and valves to provide cooling. The Joule-Thomson effect can be used in single stream cooling lines, or augmented with a heat exchanger and pump for cryogen flow on the warm side. Thermal de-stratification with any method that induces circulation of the liquid cryogen can prolong storage times by bringing the tank to equilibrium conditions (and avoiding venting). Manipulation of the boundary temperature can significantly reduce the effective environmental heat leak. This can be accomplished using a thermodynamic vent system during cryogen use, or by storing or flowing a complementary higher temperature cryogen in a jacket around the tank.
Finally, active cooling techniques require significant input power to make use of cryogenic refrigeration or cryocoolers to intercept or extract heat from the system. These refrigerators must interface with some form of heat exchanger, and perform in an integrated dynamic fashion for all required operations. These systems can also be used to subcool (densify) the cryogen for increased storage density and thermal capacity. The result is much longer storage times since significant environmental heat leak is absorbed before the cryogen reaches saturation conditions.
Design and Development
Based on the system trades and analysis performed in the previous step, each feasible design trade is ranked based upon key quantitative and qualitative criteria. Ranking methods and criteria selection should be established by consensus with the stakeholders prior to comparing design trades. This helps avoid the temptation to tweak the process in real time which can result in selection bias.
The highest ranked trade that meets all the requirements can then be selected for detailed system analysis and design. It's important during this step to document key assumptions, system interfaces, boundaries conditions, analysis approach, parametrics, optimizations, detailed results, etc. This step may require more than one iteration, or selection of the next highest ranked design trade, if the initially selected trade doesn't result in an acceptable solution.
When the design is deemed satisfactory, verification and testing is performed to validate that all the requirements are are met by the system. The project scope dictates the details of the verification and validation effort (e.g. verification of all analysis results; component and subsystem testing; and integrated system testing). The subsequent final design and system specification provides the information required to build, install and operate the system.
Optimal No-Loss Liquid Hydrogen and LNG Systems
Of course, this brief overview doesn't address all of the detailed considerations and customization needed for a specific liquid hydrogen or LNG application. However, following the overall approach helps to insure that the needs, goals and objectives are addressed in a systematic way that meets the requirements and is consistent with the concept of operation.
Furthermore, exploring the feasible trade space - and selecting the best design within it - also mitigates the risk of a sub-optimal design. Such an approach is key to economically minimizing or eliminating boil-off losses for cryogenic systems.
