--- title: "Reversed Rankine Cycle | Thermodynamics II" description: "Reversed Rankine Cycle is the heat-pump version of the Rankine cycle, moving heat from cold to hot using work input in Thermodynamics II." canonical: "https://fiveable.me/thermodynamics-ii/key-terms/reversed-rankine-cycle" type: "key-term" subject: "Thermodynamics II" unit: "Unit 6" --- # Reversed Rankine Cycle | Thermodynamics II ## Definition The reversed Rankine cycle is a vapor power cycle run backward to move heat from a low-temperature source to a high-temperature sink. In Thermodynamics II, it is the basic model behind heat pumps and refrigeration systems. ## What It Is The reversed Rankine cycle is the thermodynamics model for a heat pump or vapor-compression refrigeration system, where you spend work to move heat uphill from cold to hot. Instead of producing work like the regular Rankine cycle, this cycle uses a compressor to drive heat transfer in the opposite direction. The working fluid, usually called a refrigerant, goes through phase change so it can absorb and release large amounts of heat at nearly constant pressure. That is the big advantage of the cycle. A small amount of work can circulate the fluid and change its pressure and temperature enough to collect heat from a cool place and dump it into a warmer one. A typical reversed Rankine cycle has four main processes. First, the vapor is compressed approximately isentropically, which raises its pressure and temperature. Then it rejects heat at high pressure in a condenser or heat sink, where it condenses into a liquid. Next, it expands to a lower pressure, often through an expansion valve or throttling device in real systems, and its temperature drops. Finally, it absorbs heat at low pressure in the evaporator as it boils back into a vapor. In an idealized Thermodynamics II treatment, those steps are often described as isentropic compression, isobaric heat rejection, isentropic expansion, and isobaric heat addition. Real machines are a little messier, especially at the expansion step, but the cycle picture still gives you the energy-flow logic. A useful way to think about it is this: the cycle is not making heat from nothing. It is moving heat from one reservoir to another by adding work, which is exactly why it shows up in heat pump systems. If you are given pressure-enthalpy or temperature-entropy data, the reversed Rankine cycle lets you track where heat enters, where heat leaves, and how much compressor work is required. ## Why It Matters Reversed Rankine cycle problems show up whenever Thermodynamics II asks you to connect refrigeration or heating performance to the first and second laws. The cycle gives you a clean framework for calculating heat transfer, work input, and coefficient of performance, instead of treating a heat pump as a black box. It also ties together several big course ideas at once: phase change, entropy, enthalpy, and energy balances around a cycle. If you can follow the four state points, you can usually read a property table, sketch the process on a T-s or P-h diagram, and explain why the system needs work to move heat against the natural temperature gradient. This term matters beyond just formulas. It helps you see why household heat pumps can deliver more heating energy than the electrical energy they consume, and why the refrigerant choice and pressure levels matter. In problem sets, that often turns into comparing ideal and real cycle behavior or explaining why the compressor outlet is hot enough to heat a room. It is also a bridge to seasonal efficiency and real equipment performance, because the reversed Rankine cycle is the idealized reference point for actual machines. ## Connections ### Heat Pump The reversed Rankine cycle is the thermodynamic model underneath a heat pump. The machine uses a compressor, condenser, expansion device, and evaporator to move heat from a colder source to a warmer space. When you see a heat pump question, the cycle is the energy-analysis tool you use to track how much heating you get for each unit of work input. ### [Coefficient of Performance (COP)](/thermodynamics-ii/key-terms/coefficient-of-performance-cop) COP is the performance number usually calculated for a reversed Rankine cycle. For heating mode, it compares useful heat delivered to work input, so a COP greater than 1 is normal. That can feel strange at first, but it makes sense because the system is moving heat rather than creating it. ### Phase Change Phase change is what makes the reversed Rankine cycle efficient. The refrigerant absorbs heat while boiling in the evaporator and releases heat while condensing in the condenser, which lets it transfer large heat amounts at nearly constant temperature. If you forget the phase change piece, the cycle starts to look like a simple gas compression loop, which misses the whole point. ### [Heat Sink](/thermodynamics-ii/key-terms/heat-sink) The heat sink is the warm location where the cycle dumps heat at high pressure. In heating applications, that is often the indoor air or a water loop. In Thermodynamics II problems, identifying the heat sink helps you decide where heat rejection happens and how to label the condenser side of the cycle. ## On the AP Exam A problem set or quiz question will usually ask you to identify the four processes, sketch the cycle on a T-s or P-h diagram, or calculate COP from enthalpy values at the state points. You may also be asked to compare the reversed Rankine cycle with the regular Rankine cycle and explain why one is a heat pump while the other is a power cycle. If the question gives you a compressor, condenser, expansion valve, and evaporator, your job is to trace the refrigerant path and label where work is added and where heat is absorbed or rejected. On written responses, a strong answer uses the property language correctly: compression raises pressure and temperature, condensation rejects heat, expansion lowers pressure, and evaporation absorbs heat. The common mistake is treating the expansion step like a turbine in an ideal Rankine cycle when many real heat pump diagrams use throttling instead. ## Reversed Rankine Cycle vs Rankine Cycle The Rankine cycle is a power cycle that takes in heat and produces work, usually in steam power plants. The reversed Rankine cycle runs the other way, using work input to move heat from cold to hot. If you mix them up, check the direction of energy flow: power cycles generate work, while reversed cycles consume work to provide heating or refrigeration. ## Key Takeaways - The reversed Rankine cycle is the backward version of the Rankine cycle, and it models heat pumps and refrigeration systems. - It uses work input, usually from a compressor, to move heat from a low-temperature source to a high-temperature sink. - Phase change matters because boiling and condensing let the refrigerant transfer a lot of heat efficiently. - A standard cycle analysis tracks four states and uses enthalpy changes to find heat transfer, work, and COP. - If a problem asks about heating performance, the reversed Rankine cycle is the starting point for the energy balance. ## FAQs ### What is the reversed Rankine cycle in Thermodynamics II? It is the idealized thermodynamic cycle behind heat pumps and refrigeration systems. The cycle uses work input to move heat from a colder region to a warmer one, which is the reverse of what a Rankine power cycle does. ### How is the reversed Rankine cycle different from the Rankine cycle? The regular Rankine cycle produces work from heat, usually in a steam turbine system. The reversed Rankine cycle does the opposite, consuming work so a refrigerant can absorb heat at low temperature and reject it at high temperature. ### Why can the COP of a reversed Rankine cycle be greater than 1? Because the system is not creating all of its output heat from electrical work alone. Most of the delivered heat comes from the low-temperature source, and the work input just moves that heat to the warm side. ### What processes are in a reversed Rankine cycle? The usual ideal model has isentropic compression, isobaric heat rejection, isentropic expansion, and isobaric heat addition. In real systems, the expansion step is often throttling rather than true isentropic expansion, so instructors may ask you to notice that difference. ## Related Study Guides - [6.3 Heat Pump Systems and Performance](/thermodynamics-ii/unit-6/heat-pump-systems-performance/study-guide/Dt3As6NeysimBpLn) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/thermodynamics-ii/key-terms/reversed-rankine-cycle#resource","name":"Reversed Rankine Cycle | Thermodynamics II","url":"https://fiveable.me/thermodynamics-ii/key-terms/reversed-rankine-cycle","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/thermodynamics-ii/key-terms/reversed-rankine-cycle#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:24:17.747Z","isPartOf":{"@type":"Collection","name":"Thermodynamics II Key Terms","url":"https://fiveable.me/thermodynamics-ii/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/thermodynamics-ii/key-terms/reversed-rankine-cycle#term","name":"Reversed Rankine Cycle","description":"The reversed Rankine cycle is a vapor power cycle run backward to move heat from a low-temperature source to a high-temperature sink. 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