physics

Micro combined heat and power

Micro combined heat and power is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Micro combined heat and power rather than just read about it. In short: Micro combined heat and power, micro-CHP, μCHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 kW. Usual technologies for the production of heat and power in one common process are e.g. internal combustion engines, micro gas turbines, stirling engines or fuel cells.

Key takeaways

Micro combined heat and power belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Micro combined heat and power to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Micro combined heat and power from memory before moving on to harder problems.

Reference excerpt

Micro combined heat and power, micro-CHP, μCHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 kW. Usual technologies for the production of heat and power in one common process are e.g. internal combustion engines, micro gas turbines, stirling engines or fuel cells. Local generation has the potential for a higher efficiency than traditional grid-level generators since it lacks the 8-10% energy losses from transporting electricity over long distances. It also lacks the 10–15% energy losses from heat transport in heating networks due to the difference between the thermal energy carrier (hot water) and the colder external environment. The most common systems use natural gas as their primary energy source and emit carbon dioxide; nevertheless the effective efficiency of CHP heat production is much higher than of a condensing boiler, and thus reducing emissions and fuel costs.

Overview A micro-CHP system usually contains a small heat engine as a prime mover used to rotate a generator which provides electric power, while simultaneously utilizing the waste heat from the prime mover for an individual building's space heating and the provision of hot domestic water. With fuel cells there is no rotating machinery, but the fuel cell's stack and where applicable also the reformer will provide useful heat. The stack does generate DC power which is converted by DC/AC inverter into mains voltage. Micro-CHP is defined by the EU as less than 50 kW electrical power output, however, others have more restrictive definitions, all the way down to <5 kWe. A micro-CHP generator may primarily follow heat demand, delivering electricity as the by-product, or may follow electrical demand to generate electricity, with heat as the by-product. When used primarily for heating, micro-CHP systems may generate more electricity than is instantaneously being demanded; the surplus is then fed into the grid. The purpose of cogeneration is to make use of more of the chemical energy in the fuel. The reason for using CHP systems is that large thermal power plants which generate electric power by burning fuel produce between 40% and 60% low-temperature waste heat, due to Carnot's theorem. The temperature produced by this waste heat (around 80 °C - 150 °C) does allow it to be used for space heating purposes, therefore in some urban areas district heating networks have been installed. Heat networks have a limited extent, as it is not economical to transport heat long distances due to heat loss from the pipes, and it will not reach into areas of low population density, or else revenues per CAPEX will go down. Where no district heating is possible due to low heat demand density or because the local utility has not invested in costly heat networks, this thermal energy is usually wasted via cooling towers or discharged into rivers, lakes or the sea. Micro CHP systems allow highly efficient cogeneration while using the waste heat even if the served heat load is rather low. This allows cogeneration to be used outside population centers, or even if there is no district heating network. It is efficient to generate the electricity near the place where the heat can also be used. Small power plants (μCHP) are located in individual buildings, where the heat can be used to support the heating system and recharge the hot domestic water tank, thus saving heating oil or heating gas. CHP systems are able to increase the total energy utilization of primary energy sources. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of electricity and fuel, particularly fossil fuels, and due to environmental concerns, particularly climate change. In a traditional power plant delivering electricity to consumers, about 34.4% of the primary energy of the input fuel, such as coal, natural gas, uranium, petroleum, solar thermal, or biomass, reaches the consumer via electricity, although the efficiency can be 20% for very old plants and 45% for newer gas plants. In contrast, a CHP system converts 15%–42% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. In total, over 90% of the heat from the primary energy source (LHV based) can be used when heat production does not exceed the thermal demand. Since the year 2000, micro-CHP has become cost effective in many markets around the world, due to rising energy costs. The development of micro-CHP systems has also been facilitated by recent technological developments of small heat engines. This includes improved performance and cost-effectiveness of fuel cells, Stirling engines, steam engines, gas turbines, diesel engines and Otto engines. Combined heat and power (CHP) systems for homes or small commercial buildings are usually fueled by natural gas to produce electricity and heat. If no access to the natural gas network is available, which in general is the cheapest alternative, LPG, LNG or heating fuel (diesel) might be an alternative. The PEMFC fuel cell mCHP operates at low temperatures (50 to 100 °C) and needs high purity hydrogen. It is prone to contamination; changes are made to operate at higher temperatures and improvements on the fuel reformer. The SOFC fuel cell mCHP operates at a high temperature (500 to 1,000 °C) and can handle different fuel sources well, but the high temperature requires expensive materials to handle it; changes are made to operate at a lower temperature. Because of the higher temperature the SOFC in general has a longer start-up time and needs continuous heat output even at times when there is no thermal demand. CHP systems linked to absorption chillers can use waste heat for refrigeration. A 2013 UK report from Ecuity Consulting stated that MCHP is the most cost-effective method of utilizing gas to generate energy at the domestic level. The fuel cell industry review stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power had passed the conventional engine-based micro-CHP systems in sales in 2012.

Technologies Micro-CHP engine systems are currently based on several different technologies:

Internal combustion engines Stirling engines Fuel cell Microturbines Steam engine/Steam motor (using either the traditional water or organic chemicals such as refrigerants)

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Micro combined heat and power

Start with the simplest possible case. Write down what Micro combined heat and power claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Micro combined heat and power before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Micro combined heat and power ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Micro combined heat and power

In research: Micro combined heat and power appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Micro combined heat and power in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Micro combined heat and power is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cogeneration, Hydrogen technologies, Renewable energy, so understanding it makes those chapters shorter.
In everyday life: Look for Micro combined heat and power outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Micro combined heat and power in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Micro combined heat and power means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Micro combined heat and power out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Micro combined heat and power in simple terms?

Why does Micro combined heat and power matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Micro combined heat and power?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Micro combined heat and power.