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Hydropneumatic Accumulator with a Compressible regenerator
NSF SBIR Phase I Award (944831) This SBIR Phase I project will develop a
hydropneumatic accumulator for fluid power systems with specific applications to
hybrid vehicles. The unique aspect of the accumulator design is the leaf spring
construction being introduced into gas reservoir of the accumulator provides nearly isothermal character of gas compression/expansion processes. This in its turn leads to significant reduction of thermal losses and consequently to the accumulator power efficiency improvement. The
broader/commercial impacts of the proposed project will be in the many
application areas of fluid power systems. In the specific application to a
hybrid vehicle, there could be a fuel economy improvement. There will be a
trade-off between vehicle weight and reduced fueling benefits. However, any
efficiency in a fluid power system would translate into improved system
efficiency, resulting in broad commercial impact. Recuperated energy economy is directly
related but is not equal to fuel economy. The correct calculation of fuel
economy should take into consideration the amount of fuel already saved by using
hydraulic hybrid system in the vehicle (with a regular accumulator on board)
instead of conventional ICE system. The values of energy and fuel economy are of
the same order of magnitude, so we used energy economy as a reference. The comparative results of our preliminary testing were
obtained for three accumulator types: a conventional accumulator taken as a
reference, an accumulator filled with elastomer foam, and an accumulator with
embedded metal compressible regenerator. All the three samples were tested at
conditions close to real hydraulic hybrid vehicle drive regimes: regenerative
braking in regular urban cycle and regenerative braking at load/unload stops.
The fuel economy of incorporation
of these hydropneumatic accumulators into hybrid cars can be estimated of at
least 5%, which translates into annual savings of about 300 million dollars for
the trucks and about 10 billion dollars for the cars in the United States. This
accumulator can also be used in other hydraulic systems with high level of fluid
flow and pressure pulsations. The
Phase I will result in development and experimental approval of the methodology
for efficient design of the accumulator with optimized power recuperation
efficiency for any chosen application. The Phase II will result in development and computer verification of the methodology for efficient design of the accumulator with optimized size, form, safety factor, and weight for any chosen application. Experimental Verification For verification of the proposed
technical solution, three samples of 2 liters Hydac SK350-2/2212A6 accumulator
are taken: regular one, filled with elastomer foam and one with embedded
regenerator in the form of a multilayer leaf spring made of 120 flat leaf
elements 0.4 mm thick with sector spacers 1 mm thick between them. In this case
the stressless state of the flat leaf elements corresponds to the maximum gas
reservoir volume and for any gas compression ratio within the working range
relative deformation of the leaf elements (bending less than 1 mm with the bent
sections of about 12 mm long) is much less than the elasticity limit. The results of this comparative tests are
presented below. To verify the difference between
temperature dynamics during recuperation gas temperature variation curves were
recorded for regular accumulator and for one with embedded regenerator at three
successive power recuperation cycles of the following shape: charge - 20 sec;
hold – 50 ÷ 60 sec; discharge – 30 sec; pause – 50 sec. Initial gas pressure was
8 ± 6% MPa, compression ratio (maximal gas pressure to the initial pressure
ratio) for each cycle – 2.7 ± 6%. The ambient temperature was 18 ºŃ. To obtain power recuperation efficiency
P-V diagrams were recorded for all three samples of accumulator at the following
set of power recuperation cycles:
The detailed list of cycles is presented
in Table 1 together with power recuperation efficiency data for each cycle. Initial gas pressure in all tested accumulators is 10.5 ± 5% MPa, compression ratio for each cycle – 2.1 ± 5%. Compression ratio of 2 is commonly used for hydraulic hybrids simulation and prototypes. Compression ratio of 3 is not so common. However in this case higher energy capacity of accumulator can be achieved. So we’ve done measurements for all three samples of accumulator at compression ratio of 2.7 ± 6% and cycle shape as follows: charge - 25 sec; hold – 60 sec; discharge – 14 sec. Test Results Figures 3 - 6 give the experimental curves of the gas
temperature variation (Fig.3) and P-V diagrams (Fig.4 - Fig.6) in accumulator
gas reservoir at the power recuperation cycle with compression ratio of 2.7.
Table 1 presents power efficiency values for all power recuperation cycles with
compression ratio of 2.1. Figure 3 shows that gas in regular accumulator (without regenerator) is heated up to 106 ºŃ at the compression, cools down to 30 ÷ 32 ºŃ during the storage time, cools down to -30 ºŃ at the expansion and is heated again up to 10 ÷ 12 ºŃ during the pause. At the same time gas in the accumulator with embedded regenerator is heated up to less than 25 ºŃ at the compression and cools down to 12 ºŃ during the expansion. Embedded regenerator reduces gas heating at compression and gas cooling at expansion dozens of times, thus reducing the losses of the stored power during storage.
Figure 3. Gas temperature in reference (red) and improved (blue) accumulators for charge-hold-discharge cycles.
Power recuperation efficiency is evaluated from the P-V diagrams.
Examples of these diagrams for the cycle shape: charge - 25 sec; hold – 60 sec;
discharge – 14 sec and efficiency data are presented in Figures 4 – 6. Each
recuperation cycle gives the loop at P-V diagram where the upper curve
corresponds to the compression stage and the lower one corresponds to the stage
of expansion. The loop area characterizes thermal losses during recuperation
cycle (the wider is the loop the higher are losses and lower is efficiency). So
the difference between three accumulator samples is very clear.
Figure 5. The P-V diagram for accumulator filled with elastomer foam (gas+foam)
power efficiency 84%.
Table 1. Power efficiency of accumulator samples for different driving cycles The presented experimental data (Table 1) demonstrate substantial reduction of recuperated energy losses and corresponding increase of the recuperation efficiency in all tested regimes, when the metal compressible regenerator is introduced into a conventional piston accumulator. In conclusion, the proposed solutions allow for the creation of a hydropneumatic accumulator for fluid power recuperation with the following properties: Recuperative braking is the most well-known advantage of hybrids. Due to the highest power density hydraulic powertrains are able to absorb 100% of the vehicle kinetic energy. Depending on holding time (stopping at traffic light or loading/unloading), the recuperated energy reaches 70-80% with conventional hydraulic accumulators. When used in regular urban cycle, the metal compressible regenerator gives additional 18 - 24% of fuel economy as compared to a conventional accumulator and 7 - 8% of fuel economy as compared to a foamed accumulator. The application of the metal compressible regenerator at delivery truck cycle with 1 - 15 minutes of load/unload stops was shown to be the most efficient. In this case, an additional 25% of fuel economy can be reached as compared to a conventional accumulator and 11 - 14% - as compared to a foamed accumulator (See Table 1).
Summary of the Research Calculations of the basic design parameters, modeling of the operation, and experimental verification of the hydropneumatic accumulator with a compressible regenerator have been accomplished. High energy efficiency has been achieved by incorporation of the designed spring-like metal leaf structure (hereinafter ”Heat Spring”) into industrial hydroneumatic accumulators available on the market. The method of calculation of the gap between the metal leaves (basic Heat Spring parameter) depending on the operating cycle frequency and temperature fluctuations has been developed and used in preparation of the prototype used for further verification. The regenerator parameters have been calculated for the gap values ranging from 0.38 mm to 9.30 mm. Comparative studies using the prototype with the gap of 1 mm and a commercially available reference accumulator showed higher efficiency of this prototype for slower operating cycles. It was found that the Heat Spring design should correspond to the desired operation mode of the accumulator. The theoretical basis of the Heat Spring design provides solution to the manufacturing of the Heat Spring regenerator for specific applications. Both theoretical modeling and experimental verification show that the optimal gap value should be proportional to the square root of the compression/expansion time. The results of this feasibility study demonstrate advantages of the heat regeneration approach as compared to conventional accumulators based solely on reducing hydromechanical energy losses, with at least ten-fold reduction of thermal losses in the prototype. The Results will be Presented Stroganov A. A.; Sheshin, L. O.; Sveshnikov, N. N. Efficiency Improvement in Hydraulic Accumulators with Piston and Elastic Separators. International Fluid Power Exposition, Las Vegas NV, March 22-26, 2011 (in preparation) Aknowledgements We would like to thank National Science Foundation for its support in this research project through grant (944831). We would also like to express our gratitude to Dawnbreaker for guidance and assistance in fluid power market analysisDisclamer Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. |
Our approach addresses reduction of thermal energy losses through recuperation in the process of operation of the accumulator. This is achieved by incorporation of metal leaf structure (“Heat Spring”) in the gas chamber of the hydropneumatic accumulator (Figure 1). Heat Spring consists of a series of metal leaves (1) combined in the spring-like structure by connecting elements (2) embedded in the gas chamber of the accumulator.
Figure 1. Regular piston accumulator with the embedded compressible metal regenerator (“Heat Spring”). |
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