Compressor
1. Noise:
* Hearing damage is a health concern that is prominent to environments that exhibit large uncomfortable noise levels. Our design requires the usage of an air compressor, and this subsystem has the potential to cause such danger. The goal is to minimize the noise level as much as possible and not exceed 85dB. OSHA (Occupational Safety and Health Administration) has set regulations within workspaces, requiring employees to wear some sort of ear protection if they are exposed to a noise level of 85dB for a period of 8 hours. It is highly unlikely that the air compressor run to that extent of time. At the scale of our design, the full run time of the air compressor (filling up the pressure vessel) approximately takes 15 to 20 minutes. Taking the performance time into consideration, we decided upon the 85 dB to be the absolute maximum for the noise level. Satisfying this boundary make the feasibility of our design much more realistic when we scale it up to commercial use.
2. Mechanical Efficiency- Compressor
* The “mechanical efficiency” specification of the compressor was evaluated by comparing the work used to power the compressor and the work that is done by the air. We can calculate this using the first and second law of thermodynamics. The goal is to try to maximize this efficiency as much as possible with a minimum mechanical efficiency of 0.4, but we cannot get an efficiency of less than 0.2 because then the total system would not be worth it economically. This goal stems from research of similar products and our previous knowledge of heat transfer and thermodynamics. The typical efficiencies of commercial compressors vary but the relative efficiency of each type is as follows: Centrifugal- 0.70 to 0.85, High-Speed Reciprocating- 0.72 to 0.85, Low-Speed Reciprocating- 0.75 to 0.90, and Rotary Screw- 0.65 to 0.75 [1]. Since we are constrained by our budget, we purchased a high-speed reciprocating compressor and can use this range to compare to in our calculations. With that being said, we expect the mechanical efficiency of the compressor to potentially be lower than this range due to the compressor being commercially made for fairly cheap, so the optimization of efficiency by the manufacturer is not the main concern when producing the compressors. There are numerous uncontrollable sources of efficiency hindrance such as noise of the compressor, heat of compression, frictional losses, and motor windings losses so we must take that into account when we consider the efficiency that we are trying to achieve which is why we have a minimum target efficiency of 0.4. Maximizing the mechanical efficiency as much as possible is important because the higher the efficiency of the system, the higher the power output that the system produces. Our constraint is set at 0.2 because the whole purpose of our overall system is to take in energy and release as much of that energy as possible. If one unit of the compressor is losing 80% of the input energy, then that leaves very little energy left in the system for the output energy to be optimized within the other systems. There was some sort of losses in the other subsystems so we need to minimize our subsystem losses as much as possible. If our system is achieving an efficiency that is getting close to 0 then it is obviously super impractical and economically unfeasible.
3. Pressure Output
* The “Max Pressure” specification refers to the max amount of pressure that the pressure vessel is able to hold. The max pressure is also related to the maximum pressure that is able to be released at the output of the compressor. The goal of being able to store air at the highest pressure means that we are able to have more air in a smaller volume. Therefore this allow the system to produce more energy. The lower bound of the pressure we could have is 20psi. This is at the pressure where the air is not able to spin the motor anymore and no energy will be produced. There is a constraint though on what the max pressure of the output is and this 100psi. This constraint is because the air motor that was purchased has a max pressure input of 100 psi so we won’t be able to have the outlet pressure be over this. This doesn’t mean that the tank pressure cannot exceed the 100 psi limit of the air motor, only that we can’t supply the air motor with over 100 psi of air.
4. Airflow rate (CFM):
* The air flow rate out of the compressor has a direct impact on the mechanical efficiency of the compressor. From first law analysis, we have found that the efficiency rises as the air flow increases. Therefore, the goal would be to maximize the air flow rate and make sure it has a minimum level of 3 CFM. Calculations have shown that the favorable range for the compressor to obtain an efficiency level of 20 % starts from 7 CFM. However this necessary flow rate can be reduced with the usage of a heat exchanger that help the intercooling process between the first and second stage. Lowering the inlet temperature allow the necessary air flow rate to be reduced to 3 CFM. The minimum level was also established after considering all the different air compressors that are commercially available, and making sure that other specifications such as cost, max pressure, and tank size are not interfered. Satisfying this specification help the overall CAES system achieve its desired efficiency value.
5. Compressor Amperage/Voltage
* The “Compressor Amperage/Voltage” specification is the amperage and voltage that the commercial compressors we would have potentially purchased would require to run based on the outlets that we would be using to test our system. Our target for the voltage was to keep it drawn by the compressor at 120 V or 240 V. This would allow us to use the outlets available to us to test since most standard voltages for wall outlets are 120 V and 240 V [6]. Additionally, our target amperage was to have the current be 15 A or 20 A. This would allow us to use the outlets available to us to test since most standard amperages for wall outlets are 15 A and 20 A [7]. The compressor that we purchased can run at 120 V or 240 V and 15 A, so we were able to purchase a compressor based on this specification. This allows us to actually test our system with typical outlets that are available at very common locations, making it much more convenient. If we had to use outlets that were not standard, either we would not be able to test the system or we would have to use very uncommon ones, which would limit the potential places we could test (and potentially completely hinder our ability to test in general).
* This reasoning could also apply to the final product design for the end-user. Having our final product design running on 120/240 V and 15/20A would allow the final product to be feasible to a wider range of users. If the product needed an irregular voltage or amperage to run, then many people or groups would be less inclined to purchase the product; they either would not be able to run the system at all if they did not access to the irregular voltage or amperage or they would have to spend more money to be able to have access to this odd voltage/amperage. Using voltage and amperage at these common household/commercial values would be a goal of the final product design.
6. Compressor Cost
* The “Compressor Cost” specification is the total cost of the compressor (and pressure vessel) subsystem for our prototype, including the cost of components not included in our final design. The main component of this subsystem is the compressor itself. It does not include the cost of the manufacturing after we finalize our design. Our goal is to minimize this as much as possible and not exceed 1250 USD, while we must not exceed 1500 USD. This goal was based on the total budget we were allocated from the department and how we must split this budget for our various subsystems. Additionally, this goal was based on the available commercial compressors that potentially fit within our total budget. There were multiple compressors types that would be feasible to accomplish the goal of the project, such as reciprocating motor, rotary screw, and scroll compressors, but the reciprocating motor compressor was the most economically feasible type to fit within our budget and still allow financial flexibility. Scroll compressors can be purchased for roughly 1400 USD and above [2] while rotary screw compressors can be purchased for roughly 2900 USD and above [3], so that exceeds our budget constraint because there are other elements that we also have to buy within the compressor subteam. The reciprocating compressors (both single and double stage) were roughly 750 USD and above [4, 5], so they both fit within our budget to give us the flexibility to still buy our sensors (thermocouples and pressure transducers) and also the fittings to incorporate these sensors to be able to get our readings (among other things, such as Arduino amplifiers). We decided on the two-stage reciprocating compressor since they are more efficient than the single-stage compressor and the cost difference between the two types was pretty negligible. The reason we set the constraint as 1500 USD was because if we exceed this value then this inhibit the other teams from being able to use their budgets as well. We also want to be able to have a buffer between the subsystem cost specifications and the overall budget in case we run into any unforeseen budgetary necessities, as well as give us the ability to buy elements for the total assembly of the system.
7. Horsepower:
*
The compressor horsepower denotes the rate at which work is done by the motor. It is also the value that represents the input work of the whole compressor system. The goal would be to achieve a target value between the ranges of 0.765 HP and 2HP with a constrained value at 0.567 HP. Using the engineering toolbox [9], we calculated the required horsepower with our desired volumetric flow rate and output pressure values being the variable. For a 2 stage reciprocating compressor to achieve a pressure output of 100 psi (the minimum pressure required for the turbine to run) and a volumetric flow rate of 4 CFM (desired flow rate for increasing overall efficiency) the required HP is 0.567 HP. This is the constrained horsepower our compressor require. The desired pressure for optimal turbine performance is 175 psi. While keeping other variables constant, and changing the output pressure to 175 psi, the calculator gave us a value of 0.765 HP, which we chose to be our minimum. A higher CFM value effectively increase the efficiency of the compressor system. To increase CFM, horse power must be increased. However, larger horsepower means higher energy consumption and losses due to vibrations and friction caused by the aggressive motions of the pistons. Therefore we set a target range with the upper bound being 2HP. 2HP was calculated using thermodynamic analysis, which gives an efficiency of 10 percent. Keeping the horsepower within this range optimize the overall performance of the compressor, with other specifications taken into consideration.
8. Pressure Vessel Weight
* The “Pressure Vessel Weight” specification refers only to the weight of the compressor when first purchased. It doesn’t take into consideration any modifications that were added to the compressor after purchase. The goal we had was to minimize the weight of the compressor and the pressure vessel. The weight needed to be under 200 pounds because this appeared to be the threshold for where compressor units became stationary. Being able to have a compressor unit that was under this 200-pound threshold means that it would come already on wheels and be able to be transferred around with relative ease. This ease of transportation would allow us to be able to move the compressor around for demonstrations and make it so that the final assembly of the system could be more flexible.
Heat Exchanger
9. Heat Exchanger – Maximum Outlet Temperature
* The heat of compression is stored in the thermal energy storage vessel and then discharged through tubing to the air motor. The maximum outlet temperature during the discharging stage would ideally be as close to the heat of compression as possible. This is not feasible, however, due to heat loss to surroundings through the storage vessel or tubing, but precautions using shorter pipe length and insulation can help keep this temperature as high as possible.
10. Heat Exchanger- Pressure Drop
* Pressure Drop for the heat exchanger subsystem is the pressure loss occurring through the system of copper tubing moving from the charging process to the discharge. This specification is important because it helps identify an appropriate pump to move water through the tubing. The pressure drop was calculated using Bernoulli’s equation, operating under the assumptions: (1) the elevation was negligible, (2) flow is incompressible, (3) and steady flow. Minor head losses were calculated using a piping design that includes four 90 degree bends. Major head losses were calculated using the Reynolds number and corresponding Moody chart. The pump curve given by the manufacturer showed the flow rates that would be achievable based on the pressure drop calculated. The highest flow rate fell just under the pump curve, making the associated pressure drop the upper bound. Similarly, the lowest bound was calculated through the lowest flow rate and pump curve. Our target value falls around the middle of the pump curves from the manufacturer. It’s important to note that these values could change if we were to use a different pump based on the flow rates achievable with a new pump.
11. Pipe Length
* Considering that the energy storage container has a height of about 3 ft, this means that the total coil height of the piping must be below that. Hence, about 50 – 90 ft of piping should suffice while remaining within geometric constraints. The lower bound of piping, 15 ft, allows for the copper piping to cover the exposed piping on the compressor and a few extra feet to connect to a water pump. This lower bound would not maximize heat transfer to the thermal energy vessel. The upper bound is the max amount of piping we have available; this amount of piping allows us to coil the air and water streams into the thermal energy vessel. Our target value aims to use this amount of piping minus a few feet to account for defects.
12. Heat Exchanger – Thermal Energy Storage Vessel Size
* The thermal energy storage vessel for this heat exchanger stores the heat of compression in stationary water, and then this heat is used to charge the air motor during discharging. This specification is important because the storage vessel is a key component of heat exchanger systems and the vessel needs to house tubing for the discharging and charging phases. Its dimensions need to house the maximum radius of the coiled copper tubing, but also have to be sturdy enough to hold at least 10 gallons of water without breaking. For prototyping, the heat exchanger does not need insulation as an ideal charging and discharging cycle were completed without pause. Having a plastic storage vessel ensures there is as little heat loss to the environment as possible.
Expander/Generator
13. Expansion/Generator Noise
* The “Noise” specification is the total noise level of the prototype while running. This includes every subsystem but mainly targets the compressor and expansion systems. The goal is to minimize the sound level of the prototype and to never exceed 85 decibels. This goal was set based on the safety consideration with the human ear. 85 decibels was determined to be a reasonable value for the prototype to operate without damaging the human ear. Furthermore, OSHA has published guidelines on noise levels and has determined that sound levels above 85 decibels require hearing protection. The prototype cannot exceed 120 decibels because anyone near the prototype, while it is in operation, could severely damage their ears and eardrums from the high sound level.
14. Generator Power Output
* The “Power Output” specification represents the total power output of the generator while the prototype is running. The goal is to maximize the power output in order to run larger electrical appliances but never exceed 500 watts. This goal was set based on both the maximum power output of the generator and the maximum rpm of the generator (2300 rpm). At 2300 rpm, the power output of the generator has been measured at 500 watts and this data was published in the details section associated with the generator. We cannot allow the generator to exceed 500 watts for these exact same reasons: it is not rated for anything above those published values. The air motor’s published data specifies that it has the ability to reach 3000 rpm and this is a concern because the air motor shaft is directly coupled to the generator shaft. We control the air motor rpm by limiting the airflow to ensure it never exceeds the maximum allowable rpm for the generator.
15. Turbine Cost
* The “Turbine Cost” specification focuses specifically on the price of our primary air expansion device. Similarly to the other system components, we aim on minimizing cost to increase the cost-effectiveness of our system. Although we anticipated the turbine being one of the most expensive components of our system, we aimed for a price below $565 in order to have the necessary funds for other components. This price could not possibly exceed $1,330, which is our total budget for the expansion subsystem. In the event of reaching that constrained value, we would have to take significant shortcuts with the rest of our system. Extensive product research was done to select a cost-effective model of the expansion device. We considered many sizes of air motors at various price points in addition to larger-scale turbine-generator combination devices, which had high costs that were not worth the performance. The team settled on a 1.8 horsepower air motor, which has a satisfactory power output for our system’s goals and costs only $226 (significantly less than our upper bound).
16. Expansion- Vibrations
* The “Expansion-Vibrations” specification is the oscillations caused by the running of the air motor and generator. It is caused by the rotation of the air motor and may also apply to the air compressor which can cause vibrations. The vibrations were measured both qualitatively and quantitatively using accelerometers. Vibrations affect the mounting of the air motor and generator. The mounting for the air motor were designed in the machine shop and the heat exchanger and expansion were mounted to the same platform and had dampening pads. Understanding how to minimize the vibrations is important to inform the design choices of the mounting systems. Vibrations can weaken the joints between subsystems at a quicker rate if they are larger. Satisfying the maximum constraint of 4 m/s^2 for vibrations has informed our team’s decisions about mounting pad stiffness and helped us determine if more damping is necessary.
17. Air Motor Run Time
* Air motor selection was conducted to maximize air motor run time so that our prototype could clearly demonstrate its ability to supply electric power to our stakeholders, who include EV manufacturers, institutions looking for energy storage systems, and future researchers. We want all of these stakeholders to see the potential CAES has to be integrated into energy grids, so demonstrating its ability to supply consistent and sustained power was an important metric for our overall design.
Air motor selection was made with maximizing run time in mind. Using the fixed volume and max pressure output of the compressor, we used power (hp) and maximum discharge rate (cfm) data from our potential motors to calculate theoretical run times.
* Thirty seconds was decided upon on the lower bound of desired run time so air motors with theoretical run times less than that bound were not serious contenders. Our final air motor choices both exceed the lower runtime threshold, so the final design decision was made based on other parameters such as price and power output.
* A crude experiment was conducted where the compressor was charged and discharged into the air motor using only the provided hose output of the compressor. The motor’s shaft spun for 510 seconds (8.5 minutes). While this experiment neglected potential losses that would be experienced through the heat exchanger and other connections, it was encouraging to see that the selected hp and discharge rate allowed for a prolonged run time.
18. Generator RPM
* Finding a traditional generator that could accept variable input motor speed was difficult, so when talking to Professor Helm he suggested looking into air motor generators that are used on naval wind turbines. These generators are made to accept wind-varying inputs and have the ability to output significant power. However, because they are designed to work with larger and slower wind turbines, many generators we were finding delivered max voltage at low rotations per minute.
Generating maximum voltage at lower RPMs has a drawback on electrical current output. Windzilla writes in their generator specifications about this effect, “ […] the amp output is not impressive because, in order to ramp up the voltage, thin wires must be used in the winding of the alternator which impacts the output in strong wind.”
To reach a high and sustained power output, higher current flow out of the generator is desired. Because of this, wind turbine generators with higher RPM values were favored. Below is a datasheet from Windzilla that shows it reaching 12V at 540 RPM.
19. Output Demonstration Method
* The “Output Demonstration Method” specification refers to our system’s ability to demonstrate its range of power output in an effective and entertaining manner. Since the system’s generator produce varying electrical power ranging from 0 to 500 watts, dependent on the pressure of the compression vessel, it is beneficial to use its electricity to power something that visually depicts a change in power. Ideally, the device being powered should be capable of using up to 500 watts so that electricity produced by our system is not wasted. The device must be compatible with DC current, as that is what is being output by our generator. We also neglected the idea of using our system’s output to power an electric fan, as using rotational power to create electricity to create rotational power seems trivial for our goals. Our current idea is to power an electromagnet that applies magnetic force on a spring, allowing us to visualize and measure the elastic potential energy created by our system throughout its demonstration.
Overall System
20. Prototype Cost/ 21. Commercial Product Cost
The “Prototype Cost” specification encompasses the total price of every material and part purchased for our system. This does not include the cost of manufacturing time in the shop or the cost of measurement/analysis tools. The goal with this specification is to minimize the total cost of this system and to not exceed $2,700. This value was determined by performing initial product research and estimating the general cost of each component, which was then incorporated into our initial budget argument. While we aim to not exceed this value, we would be able to convince the department to increase our budget to around $3,200 if necessary. Minimizing cost is a very important aspect of our design, as our system is meant to be a small-scale model of much larger, more expensive CAES systems. The process of minimizing cost relies heavily upon conducting research to find less expensive models of each component without sacrificing performance. For example, we settled on a smaller air motor with a slightly lower power output compared to larger, more powerful models that came with extra functionalities and higher price tags. We also opted towards designing our own system mounts made of aluminum and wood, rather than purchasing premade mounts. Finding less expensive, yet effective parts and materials is vital for resulting in a low overall prototype cost that matches the small-scale applications of our system.
Our commercial cost encompasses the above information in the prototype cost but follows the total design project budget limit of $4000 that must be met when purchasing equipment.
22. Roundtrip Efficiency
The roundtrip efficiency is a ratio of the power output over the power input. The ideal power output is 500 W to power something as simple as an electromagnet or train. The compressor subsystem gives and idea as to what the power input is. At a minimum, the wattage inputted by wall outlets is 1800 W and at a maximum it is 4800 W. This yields the upper and lower bounds where the constrained value is the lower bound because this assumes max power output over max power input.
References
1.“How to estimate compressor efficiency? Campbell tip of …” [Online]. Available: http://www.jmcampbell.com/tip-of-the-month/2015/07/how-to-estimate-compressor-efficiency/. [Accessed: 24-Feb-2022].
2.“DynAir Air Compressor Silent and Oil Free, 1100W,” Net32. [Online]. Available: https://www.net32.com/ec/dynair-air-compressor-silent-oil-free-1100w-d-144463?utm_source=google&utm_medium=cpc&adpos=&scid=scplp144463&sc_intid=144463&utm_campaign=bl_shopping_high_value_products_new&gclid=CjwKCAiAgbiQBhAHEiwAuQ6BkrkdGN-kHqysWjLZUibVFZ75ynO-xxdGqjitarotP5AcN0ZJnTvk8BoCP5kQAvD_BwE&gclsrc=aw.ds. [Accessed: 24-Feb-2022].
3.“37.4CFM 116 psi rotary screw air compressor 460V 3 phase 10 hp,” Toolots, Inc. – Reliable Equipment Fast. [Online]. Available: https://www.toolots.com/air-compressor-c8822-ag-10a-460v.html?cid=1469602989&gclid=CjwKCAiAgbiQBhAHEiwAuQ6BkqxDW64gv5eXedZ32WEM1E7GJDHL5pQ3hz-5SVZ-HzeiRMND_2RC1xoCPoMQAvD_BwE. [Accessed: 24-Feb-2022].
4.“Campbell Hausfeld 2-stage 30 gal. Portable Electric Air Compressor XC302100,” The Home Depot. [Online]. Available: https://www.homedepot.com/p/Campbell-Hausfeld-2-Stage-30-Gal-Portable-Electric-Air-Compressor [Accessed: 24-Feb-2022].
5.“SS3L3 small reciprocating air compressor,” SS3L3 Small Reciprocating Air Compressor – 32334005 from INGERSOLL RAND | Acme Tools. [Online]. Available: https://www.acmetools.com/ss3l3-small-reciprocating-air-compressor-32334005/409135000181.html?feeds=shopping&srsltid=8981b0caf285434f8821256b4be70a58. [Accessed: 24-Feb-2022].
6.“How to tell the voltage of an outlet,” Hunker. [Online]. Available: https://www.hunker.com/13414185/how-to-tell-the-voltage-of-an-outlet. [Accessed: 24-Feb-2022].
7.“Electrical outlet types,” The Home Depot. [Online]. Available: https://www.homedepot.com/c/ab/electrical-outlet-types/9ba683603be9fa5395fab904ae3e00b. [Accessed: 24-Feb-2022].
8.“4AM-NRV-130,” Gast Manufacturing. [Online]. Available:
https://gastmfg.com/products/4am/4am-nrv-130. [Accessed: 28-Feb-2022].
9. “Horsepower required to compress air,” Engineering ToolBox. [Online]. Available: https://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.html. [Accessed: 20-Feb-2022].