Understanding Stirling Engines

[Music] advanced power systems sterling engines generating power in space is a critical component of survival complete loss of power in space is almost always fatal the efficiency of your power generation system will determine how much of the available power you can use understanding power generation systems will be vital to your selection and success in the space industry first let's make sure that we are on the same page when we talk about power we will define power energy work and force then we will apply our understanding to a power generation system power is the amount of energy transferred over a period of time the unit of power is the what the unit of energy is the joule one joule of energy transferred over one second is a watt a watt is also defined as kilogram meter squared over seconds cubed let's relate this to our previous lessons the fundamental elements of matter are mass and velocity einstein taught us that these are relative at extreme speeds near that of light increased velocities will increase our effective mass sadly we don't have any ship fast enough to make this a major issue so we will stick with newton for now if we start with mass and velocity we can multiply them and get the momentum of an object if an object is moving at a constant velocity it will have a defined momentum this is reported in kilogram meters per second if there is a change in velocity then the object is accelerating or decelerating which is just negative acceleration and we have to track the change in velocity over time anytime there is a change in an object's velocity we can use the fundamental elements of that matter to know the force resulting in that change if an object has a mass of 10 kilograms and was moving at 5 meters per second but accelerated to 10 meters per second over 5 seconds we know the momentum started at 50 kilogram meters per second and ended up at 100 kilogram meters per second but how much force was applied to create this change mass times delta v over delta t will give us the answer now we know the force that must have been applied to bring about this change in acceleration and now that we understand momentum and force let's look at work work can be thought of as the change brought about by applying force if there is a one ton cube of platinum i have mind from the moon and i push against it to move it one of two things will happen it will overcome the friction against the floor and move or it will not if i have a mass of 100 kilograms and i run at the block at 10 meters per second hitting it with my outstretched hands and pushing as hard as i can coming to a complete stop within one second then my mass times my change in velocity over time will give the force that i have applied to the block 100 kilograms is still my mass i went from 10 meters per second to zero meters per second over one second so my deceleration was 10 meters per second squared multiplied by my mass of 100 kilograms i get 1000 kilogram meters per second square or 1000 newtons now when i push on the cube it pushed back with an equal and opposite force as long as it didn't collapse the mass of the cube resists a change in velocity we call this inertia and when a cube is sitting on a floor we have to factor in something called static friction this is the friction force that resists the cube moving at all it is greatest just before the cube starts to slide once the cube is sliding we must still overcome sliding or kinetic friction but it is always less than the static friction the friction depends on the surface a smooth metal surface for sliding boxes is better than a rough surface the area in contact and the weight of the object are also factors to consider if we are in free fall there is no force of weight pushing the cube to the surface so the cube is free to lose contact with the floor if we are on the moon the force of gravity there creates an acceleration of 1.33 meters per second squared compared to the earth with an acceleration of 9.81 meters per second squared since friction depends on the object's mass and the force of gravity or its weight we can slide a much larger cube on the moon than the earth if everything else is the same but we have to remember our weight on the moon is less also so when we push on the cube it is easier for the cube to push us backwards now if the cube moves then we can say that work has been done if the cube slides 10 meters from my efforts we say that the work performed was force times distance so 1000 newtons times 10 meters gives us 10 000 newton meters of work the unit of work is newton meters or kilogram meters per squared over second squared which is also a joule this is the standard unit for energy a joule is the amount of energy required to move something one meter against one newton of force if the cube does not move then no work was done this always confused me i still applied a force where does that go the force i applied to the cube if it did not move the cube was insufficient to overcome friction and the cube absorbed the force as heat energy so remember when you apply force to an object you either move it or you don't if it moves some of the energy is transformed into work if it does not move that energy is transformed into heat that's right the molecules of the cube absorb and distribute the force and there is a tiny transfer of heat if you doubt this take a hammer and beat an anvil resting on concrete use a digital thermometer to record the temperature of the anvil hit it 10 times really hard then record the temperature again at the impact site or just feel it carefully with your hand you will quickly verify that the anvil is hotter so now we have defined momentum force work and energy we know that a change in velocity creates a force force applied to an object transfers energy that energy creates work if the object moves and heat if it does not in actuality there is some heat generated even if the object does move this is why a moving machine heats up if we had a piston engine that ran on compressed gas it would still heat up every engine generator or other device has heat loss as energy that could go to work is lost in heating of the components of the engine let's look at energy a little further then we will look at heat more closely there is such a thing as potential energy if i am on the moon and there is a large boulder on the edge of a precipice it has a potential energy equal to its mass times the height above the ground times the force of gravity the formula is mgh or mass times gravity times height if the boulder has a mass of 10 000 kilograms and the acceleration due to gravity on the moon is 1.33 meters per second squared then we get a force of 13 300 newtons for weight multiply this times the height of let's say a hundred meters and we get one million three hundred thirty thousand newton meters or one million three hundred thirty thousand joules of potential energy this would usually be reported as one point three three mega joules if i attach a spool of cable to this boulder and put the spool on a generator so that the force of the falling boulder will spin my generator i can calculate how much energy i will get from my generator if my generator is 90 efficient then i will get 1.33 megajoules minus 0.133 megajoules equals 1.97 megajoules we round this to 1.20 megajoules to maintain significant figures if i use the power produced by my generator to charge the battery on my suit and my charging efficiency is 95 i will lose 0.06 megajoules and end up with 1.14 megajoules of energy in my battery how long will that keep me alive if it takes 100 watts to power my suit and we know that a watt is one joule per second then we can divide my 1.14 megajoules or 140 000 joules by 100 watts and get 11 400 seconds divide that by the 3600 seconds in an hour and we get about 3 hours and 17 minutes that i can power my suit so the potential energy of the boulder was turned into electrical energy as it fell by using my cable and generator the generator would have slowed the boulders fall assuming its momentum came to zero just as it touched the ground normally the boulder would have its potential energy converted into kinetic energy let's say my cable snaps as soon as the boulder starts to move now the boulder hits with the same energy that it started with it started with 1.33 megajoules of energy and it ends with the same transferring this energy into the point of impact we can calculate the velocity it is going when it hits by using the formula for kinetic energy kinetic energy equals one-half the mass times the velocity squared this means the velocity equals the square root of two times the kinetic energy over the mass we know that energy is conserved so the energy the boulder impacts with is the same as the potential energy it started with before it fell since there's no air resistance on the moon we don't have to worry about that so potential energy equals 1.33 mega joules and kinetic energy equals 1.33 megajoules how much force will the boulder impact with that is determined by how long it takes to stop if the ground was soft and the boulder took a full second to stop the force imparted would be different than if the ground was harder it took a tenth of a second to stop force equals mass times acceleration acceleration equals change in velocity over change in time if it stops in one tenth the time the force imparted is ten times more the deformation and heating of the impacted ground are also factors a vehicle bumper or an airbag can be so effective because spreading a change in momentum over a longer time dramatically reduces the force imparted now to understand sterling engines let's look at a different type of work heat is just what we call the kinetic energy of subatomic particles in a solid the particles will bound in place but vibrate faster when heated and slower when cool all vibrating particles emit infrared radiation because a moving electrical charge creates a magnetic force that is transmitted by the electromagnetic force carrier the photon that means that even in the vacuum of space objects are radiating thermal energy by giving off infrared photons if something gets really hot the frequency of these photons goes up reducing the wavelength and the photons given off can have enough energy to be visible light that is why a heated piece of iron becomes hot but you can't see it then it becomes red then yellow then white as it gets hotter and the frequency increases and the wavelength decreases now we understand power energy force momentum and some of the fundamental characteristics of matter let's apply what we have reviewed to power production in space power is a critical necessity in space unlike the earth it will be impossible to live anywhere except the most massive ships and colonies without artificial life support even these will need constant monitoring and adjustments what is the best way to generate power in space we all know solar energy is readily available in space until you get past the orbit of mars solar will almost always be an important component of most space missions now that the efficiency is high enough to be practical when the viking landers went to mars the efficiency of solar panels was not sufficient to justify using that as a power source instead they depended on radioisotope thermal generators which we studied at length in another course rtgs generate heat and this heat is converted to electricity usually through a germanium silicone thermocouple this makes a solid-state power source that is very dependable but the inefficiency of this design limits its power output a different method of turning heat into electricity is being used in several designs will almost certainly have a place in the future of space exploration and colonization this is the sterling engine the sterling engine was invented by robert sterling and patented in 1816. this engine was very efficient and employed a closed cycle design this is very different from other heat engines like steam engines that depend on air and water exchange with the environment you can't really use these other engines in space where air and water are precious commodities this unique design was called a regenerator by 1843 one of these engines was driving all the machinery at an iron foundry steam engines became safer and more efficient and those sterling engines faded from use over time electric motors also became available with publicly available electricity the sterling engines mostly disappeared then in 1951 new materials and engineering solutions were applied to the sterling engine and the 200 watt generator was produced by phillips these engines became more efficient and their closed cycle function brought them to the attention of nasa which started a sterling technology branch this has brought about several excellent sterling engine designs for use in space let's take a look at these to understand sterling indians we must understand what entropy is and how it relates to heat entropy is a measure of the order of a system a random system has low entropy while a highly ordered one has high entropy if we look at the sun as an example we note that the sun is a highly concentrated mass of hydrogen helium and energy it is a fundamental principle of our universe the energy flows from high entropy locations to low interference just like a stone rolling down the mountain will continue falling until it is stopped by an obstruction or the ground the energy in the sun tries to escape and spread itself into lower energy areas like the space surrounding the sun matter and energy stream from the sun in massive amounts heated or pressurized gas expands and creates a pressure differential with the gas outside the device it tries to relieve this pressure by spreading into the environment if we put a gas under a higher pressure it contracts in volume and heats up if we put a gas into a lower pressure area it expands and cools down this is how most air conditioners and refrigerators work they take a gas like freon let it expand in a radiator in your refrigerator or house the expanding gas is colder than the air around it and the gas absorbs heat the gas is then sent outside your house refrigerator where it is compressed causing it to heat up to a temperature higher than the surrounding air this causes the heat absorbed by the gas when it was in your refrigerator to spread into the environment this is a heat pump it takes heat from inside and puts it outside it uses electricity to run a compressor there is a high pressure line running to a radiator inside and a low pressure line returning to a radiator back outside you will find that the higher the temperature outside the less efficient your system is and the harder it is to work that's why a spray of water on a radiator cools it down and increases the efficiency of device as some of the heat energy is carried away in evaporative cooling jet engines burn fuel and oxygen in a turbine at the back of the engine the escaping gas hits the blades attached to a shaft and causes the shaft to spin the shaft has another set of compressor blades on the front and sometimes a large fan so momentum is transferred to the compressor in the back which spins the compressor in the front the fan and forward compressor bring air into the engine and compress it before mixing it with fuel in the rear compressor and allowing it to expand continuing the cycle military fighter jets don't have a big fan because while being more efficient it would slow them down the same principle works to generate power a jet turbine can be used to turn a shaft connected to a generator or even a car drive train some cars in the 1960s had experimental jet engines to spin the drive shaft and direct power to the wheels they were not very efficient and piston-based internal combustion engines work better the overall propulsive efficiency of a system is described as eta equals a to c times a to p where ada c is the cycle efficiency and a to p is the propulsive efficiency the cycle efficiency is the percentage of the potential heat energy of the fuel that is converted into mechanical energy the propulsive efficiency is the proportion of the mechanical energy that can be used to actually propel the device all engines have inefficiencies and lose energy through heating parts of the engine that don't need it friction between the engine parts and between the gases and the engine and other ways for rocket engines this value for overall propulsive efficiency can be as high as 70 percent not as high as the 86 percent for propeller engines but that's at low speed at high speed or high altitude the turbo fan is better up to about mach 1.6 in the jet engine without a large fan remember this is propulsive efficiency how well does an engine propel something through the air this varies for all aircraft and rocket engines by altitude and speed air breathing engines get less efficient with altitude as the air gets thin rockets get more efficient with altitude as the pressure outside the nozzle drops and the exhaust velocity can increase heat efficiency is different from propulsive efficiency thermal efficiency calculates how much of the heat available to an engine is turned into usable work energy and heat minus loss equals energy out in work energy out over energy in equals the thermal efficiency we already said that the rtg has a thermal efficiency of three to seven percent a nuclear reactor is about thirty percent which is why the kilopower project wants a reactor with sterling engines rather than an rtg with thermocouples the thermal efficiency of a sterling engine can be as high as 30 percent that of an internal combustion engine is usually as we said at best 25 percent though the new advanced cryogenic evolved stage hydrogen burning internal combustion engine designed by ula is said to have a thermal efficiency of up to 40 percent this is because the cold hydrogen gas boiling off the tanks can be used to cool the engine while the hydrogen oxygen combustion cycle is one of the most energetic chemical reactions we have where does all this leave us if we trap this heated gas in a piston we can use the force of its expansion to work for us the difference between an internal combustion engine and a sterling engine is that the internal combustion engine takes in air for auction to burn with fuel then expels the exhaust into the environment this won't work well in space the sterling engine uses two chambers one hot and one coal that has a gas sealed inside it and a regenerator or heat exchanger between the engine pumps heat from the hot end to the cold end putting pistons in between to make the heat energy accomplish work the heat energy is converted to mechanical energy by the piston system and the mechanical energy is converted into electricity by putting coils and permanent magnets in the right place on the engine in summary if we heat one end of a sterling engine the gas sealed in the engine will expand and move a piston compressing the gas at the other end of the piston this causes that area to radiate heat and then cool down the compressed colder gas will then be allowed to flow around to the hot end which is now expanded and cooled the cold gas absorbs heat and starts to expand and the cycle starts all over let's watch this video from amtec sunpower showing a sterling based heat pump used for cryogenic cooling the principles are the same for power generation and this device electrical power is applied to move heat against the gradient and cool something down sun power incorporated cryotel cryo coolers utilize the sterling thermodynamic cycle to convert motion into a transfer of heat first electricity is introduced via the feed-through pins and travels to the stator assembly the alternating flow of electricity creates a magnetic field around the stator assembly which interacts with permanent magnets attached to the piston assembly and causes the piston to oscillate the piston's movement creates a pressure wave that causes the displacer assembly to move the displacer rod connects the assembly to a planar spring that's designed to resonate at the cryo-cooler operating frequency the cryo-cooler's moving parts are contained inside a hermetic enclosure filled with helium gas which eliminates the need to frequently recharge the cryo coolers sunpower cryo coolers contain patented gas bearing technology that harnesses the pressure wave generated by the piston funnels it through a one-way valve and out several locations on the piston outer diameter the same gas bearing technology is also used in the displacer this keeps the piston and displacer centered in the cylinder achieving contact free operation and enabling a long life the movement of the piston and displacer can cause unwanted vibrations but these can be significantly reduced with the active or passive balancer the key to achieving the cooling effect is the relationship of the movement between the piston and displacer when the displacer movement results in the bulk of the working gas being located between the displacer and cold tip the piston expands the gas based on the ideal gas law and the relationship between pressure volume and temperature this causes the temperature of the gas to decrease as the gas temperature decreases it absorbs or lifts heat through the cold tip causing the cold tip temperature to also decrease when the displacer movement results in the bulk of the working gas being located between the displacer and piston the piston compresses the gas again based on the ideal gas law this causes the temperature of the gas to increase as the gas temperature increases it transfers or rejects heat from the gas through a heat exchanger to the environment inside the displacer is a porous metal structure called the regenerator the regenerator alternately stores and releases the heat of the helium gas to drastically increase the cycle efficiency since this cycle occurs 60 times per second the net effect is that the temperature of the cold tip drops to negative 230 degrees celsius or 40 kelvin giving sunpower cryotel cryo coolers a wide range of applications this engine can be used as a refrigerator by using electrical energy to pump heat away from something and make it colder than the environment when i discussed carbon dioxide batteries in a previous course i should have emphasized that any energy system to function must have a gradient or differential no matter how much heat you have if you don't have a cold place to move it to you won't get energy from the system when charging our carbon dioxide batteries on mars we have to have somewhere with less carbon dioxide to put it so we can discharge the batteries our gravity battery quits working when our weight hits the ground and our heat engine quits working if both sides of the sterling engine are the same temperature why can't we just use thermocouples like the rtg we talked about the simple answer is inefficiency the rtg generates heat by the decay of a radioactive element and uses a thermocouple to generate electricity this only turns about three to seven percent of the available energy into electricity this is a terrible inefficiency sterling engines have been able to achieve an overall 9.8 thermal efficiency so a 40 to 300 percent increase in the power we can get from the same amount of heat the downside is that while the rtg is solid state and can operate reliably for decades a sterling engine has moving parts and while very dependable would most likely have a higher rate of malfunction sterling engines are being used in the kilopower project to generate electricity for use in space [Music] nasa is advancing an existing technology to enable future space exploration of the solar system with plans to expand human missions to mars and science exploration of the outer planets the need for reliable power becomes essential space qualified nuclear reactors are one technology that can provide for safe and reliable power for many of these missions nuclear power in space is not new these clips from the 1960s provide a window into the past a u.s program was put into place that developed and flew the world's first space reactor [Music] before the reactor is turned on that is prior to fission starting the reactor fuel is very safe and only mildly radioactive a joint venture between nasa and the department of energy is underway to develop a new space reactor that meets a range of exploration missions the reactor called kilopower can deliver a range of 1 to 10 kilowatts of electricity that is enough to power anything from one toaster to an entire household has been tested in the nevada desert at the nevada national security site the test had a fissioning reactor deliver heat to sterling converters each converter produced about 100 watts the goal of the test was to confirm the system's predicted performance the reactor core is a cylinder of enriched uranium that is six inches in diameter a beryllium oxide reflector will surround the uranium core a single rod of boron carbide is used to turn on the reactor the reactor uses well-established nuclear physics to self-regulate the fission reactions and this feature eliminates the need for a complicated control system the reactor uses nuclear fission to produce heat which is delivered by heat pipes to power generators known as sterling converters a radiator is used to keep the sterling converters cool [Music] a great deal remains to be done but with the successful completion of the nuclear test in nevada nasa is coming ever closer to the reality of a space-qualified nuclear reactor over the coming years the reliability and safety of kilopower will be tested to assure that when the new era of space exploration begins all systems will be a go [Music] this device actually has a reactor with one carbon rod to adjust power output beryllium to reflect neutrons heat pipes to carry heat away from the reactor and eight sterling engines to generate electricity from the heat [Music] face science missions to explore the underlying fabric of the universe these missions provide data on the giant gaseous planets such as jupiter and saturn their moons and other solar system objects such as asteroids and comets these missions are vital to understanding the origins of the universe historically deep space missions have relied on radioisotope power systems for energy because solar power is not practical beyond mars the nuclear isotope that powers these systems is plutonium-238 nasa has used plutonium-238 systems developed in part by the los alamos national laboratory safely and reliably for over 40 years currently the united states has recognized that there is a potential shortage of plutonium-238 and the department of energy is restarting production of this valuable isotope to enhance nasa's exploration capabilities los alamos is proposing a very small uranium based reactor as a supplement to radioisotope power systems for some deep space missions this will reduce the demand for plutonium-238 and allow it to be available for missions where radioisotope systems are the only practical option the small reactor system proposed by los alamos is very simple and relies on many well-established technologies the proposed reactor is made up of six pieces a reactor core core reflector a rod to start the reactor heat pipes to move energy radiation shielding and sterling engines for power the power source is a 50-pound cylinder of enriched uranium that is approximately four inches in diameter a beryllium reflector that is approximately 10 inches in diameter surrounds the uranium core the entire reactor core is about 12 inches tall a single rod of boron carbide is used to turn the reactor on or off the reactor uses well-established nuclear physics to self-regulate the fission reactions and eliminates the need for complicated control systems eight heat pipes that are embedded in the beryllium reflector remove heat produced in the reactor core heat pipes are passive heat removal devices that efficiently move thermal energy heat pipes were invented at los alamos in 1963. since then heat pipes have gone mainstream and are used in applications ranging from electronics to the alaskan pipeline if you own a laptop its cooling system likely uses heat pipes heat pipes remove the need for complicated coolant loops and pumps in this reactor the heat pipes connect to eight sterling engines that produce the electricity for the spacecraft sterling engines initially developed in the 19th century are becoming a very reliable and robust power conversion technology particularly for low power applications both commercially and as part of nasa's advanced sterling radioisotope generator the proposed small reactor could produce 500 watts of power from eight sterling engines weighing about three pounds each a radiation shield lies between the reactor core and the sterling engines the shield protects the electronics in the sterling engines the spacecraft and scientific instruments the small reactor system proposed by los alamos would have many advantages the design is simple and relies on existing technologies it would be very safe and very easy to manufacture it would also rely on materials that the united states has in abundance taking this reactor from the drawing board to a launch into space would be easier than most nuclear systems because the reactor doesn't operate until it is in space accidents on the ground or during launch have minimal public impact the use of uranium fuel and the low power of the reactor mean that safety and regulatory issues are greatly reduced or eliminated the simplicity of the system makes design and testing straightforward los alamos sees this small nuclear reactor as a stepping stone to a reinvigoration of nuclear reactors for space applications this small reactor could be scaled up to more challenging applications like surface power sources on mars or spacecraft propulsion using electrically driven ion thrusters scientists and engineers have always acknowledged that the future of many space missions will only be feasible with reliable and safe nuclear energy now is the time to take a bold step into the future one of the most amazing uses of sterling technology is in the gotland class of submarine developed by the swedish navy this submarine uses diesel fuel and liquid oxygen to generate heat either through two 970 kilowatt diesel engines when on the surface or through two giant 75 kilowatt sterling engines when submerged they use the liquid oxygen to power the combustion process this combustion generates heat and the pistons move the heat to the other end of the device that is cooled by the ocean the device moves a wire through a magnetic field creating electric current this generates electrical power to power and move the submarine the colder ocean is the heat sink into which the heated energy is pumped the incredibly quiet and reliable sterling engines have made this one of the stealthiest submarines on earth actually defeating a u.s navy exercise to detect it and prevent it from approaching an american super carrier now i am a firm believer in applying existing technology to use in space before reinventing the wheel sterling engines would work well on the moon for backup power when the sun is up during the 15 earth day lunar day which i think should be called one loon or something perhaps sun moon and dark moon or something like that at any rate the technology of submarine design is almost exactly what we need for the first true spaceships they operate for months in a semi-closed environment they of course have an endless supply of water and access to air while we will have to recycle our most designs for nuclear-powered spaceships show large metal heat radius i think large gas-inflated heat radiators should also be considered cryogenic propellant could be pumped around the cold end of the sterling engine then into the inflated radiator as the heated gas cools it could be pulled from the end to be fed through the engine for propulsion or power generation the added efficiency of nuclear reactors over rtgs and sterling engines over thermocouples make this combination especially important for the future of space colonization thank you very much for listening don't forget to like and subscribe we will be adding a patreon page soon for those who wish to contribute take care and be safe you

Channel: Terran Space Academy

Views: 2,819

Rating: 4.7477479 out of 5

Keywords: Stirling Engines, Power Generation, Power for Space Exploration, Power for Space Colonization, Understanding Power, Energy, Force, Work, Momentum

Id: KKnu73DJbEQ

Length: 32min 6sec (1926 seconds)

Published: Wed Jul 22 2020