Recent Trends in Waste Heat Recovery

What does the term 'Waste Heat' mean?

In a typical developed country as much as 40% of total fuel consumption is used for industrial and domestic Space Heating and process heating. Of this, around one third is wasted. This wasted heat can be lost to the atmosphere at all stages of the process; through inefficient generation, transmission or final use of that energy.

What is the aim of it's recovery?

Waste heat recovery aims to minimize the amount of heat wasted in this way by reusing it in either the same or a different process.

Waste heat can be recovered either directly (without using a heat exchanger—e.g., recirculation) or, more commonly, indirectly (via a Heat Exchanger).

Direct heat recovery is often the cheaper option but its use is restricted by location and contamination considerations.

In indirect heat recovery, the two fluid streams are separated by a heat transfer surface, which can be categorized as either a passive or active heat exchanger. Passive heat exchangers require no external energy input (e.g., Shell and Tube, Plate, etc.) whilst active heat exchangers do (e.g., thermal wheel, Heat Pump, etc.).

A survey conducted on waste heat recovery shows the above results. At an average, Only 34% of energy produced is efficiently utilised in various systems such as factories, automobiles, power plants etc.

Now, don't you think the system has very low efficiency? Shouldn't some part of rest 66% be utilised to sone extent, let's say atleast 50-50?

When considering waste heat recovery, the key question is always that of financial justification: "How much money will be saved?" The decision to recover waste heat depends critically on whether the resulting energy cost savings outweigh the installed cost of the proposed waste heat recovery project. As a general rule of thumb, a waste heat recovery project is unlikely to be installed if its payback period is longer than two or three years.

Can the waste heat be further classified?

When describing waste heat recovery it is important to specify the nature of the waste heat in terms of temperature and material phase. Waste heat can be considered as either low grade (<100°C), medium grade (100°C–400°C) or high grade (>400°C).

But how does an industry decide if it’s worth to recover the heat wasted? I mean, there might be some parameters for it to be decided!

Grades of Waste Heat

Is the recovered heat at a sufficiently high temperature to be useful? If not, would a heat pump be appropriate?

Requirement for Recovered Heat

What are the requirements for the quantity of heat recovered? Can the recovered heat be put to good use?

Cost of Transfer from Source to Sink

How much will it cost to transfer heat from the source to the sink? Would a run-around-coil be more cost-effective? Does the heat source stream contain any contaminants?

Supply and Demand Coincide

Do the patterns of heat availability (at source) and use (at sink) coincide, in terms of both quantity and timing? Is the cost or option of storage acceptable?

Existing Techologies for waste heat recovery

Waste heat found in the exhaust gases of various processes or even from the exhaust stream of a conditioning can be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat.

Some of other used techniques are:

Recuperators: This name is given to different types of heat exchanger that the exhaust gases are passed through. The heat wheel is an example which operates on the same principle as a solar air conditioning unit.

Regenerators: This is an industrial unit that reuses the same stream after processing. In this type of heat recovery, the heat is regenerated and reused in the process.

Heat Pipe Exchangers: Heat pipes are one of the best thermal conductors. They have the ability to transfer heat hundred times more than copper.

Thermal Wheel or Rotary Heat Exchanger: consists of a circular honeycomb matrix of heat absorbing material, which is slowly rotated within the supply and exhaust air streams of an air handling system.

Economiser: In case of process boilers, waste heat in the exhaust gas is passed along a recuperator that carries the inlet fluid for the boiler and thus decreases thermal energy intake of the inlet fluid.

Heat Pumps: Using an organic fluid that boils at a low temperature means that energy could be regenerated from waste fluids.

Now as this heat is recovered, there must be something that could again convert it to power, which can be used. These units are called as Heat to Power units.

Some of exisiting heat to power units are-

An Organic Rankine Cycle unit uses an organic fluid as the working fluid.

Thermoelectric unit (Seebeck, Peltier, Thomson effects) units may also be called WHRU, since they use the heat differential between two plates to produce DC Power.

Shape-memory alloys can also be used to recover low temperature waste heat and convert it to mechanical action or electricity.

Now, all the basic overview abiut the recover might be clear. Let's just see the advantages and disadvantages of this system.

Advantages

Direct benefits

• The recovery process will add to the efficiency of the process
• Decrease the costs of fuel and
• Energy consumption needed for that process.

Indirect benefits

• Reduced Pollution
• Reduced equipment sizes
• Reduced auxiliary energy consumption

Disadvantages

• Capital cost to implement a waste heat recovery system may outweigh the benefit gained in heat recovered.
• It can be difficult to efficiently utilize the quantity of low quality heat contained in a waste heat medium.
• Heat exchangers tend to be larger to recover significant quantities which increases capital cost.
• Maintenance of Equipment: Additional equipment requires additional maintenance cost.
• Units add size and mass to overall power unit.

Future Trends in this domain

Some of current researches and R&D developments are based on the following processes which are likely to be used in near future:

• Thermochemical Recuperation of Fuel Gas
• Water Vapor Condensation in Exhaust
• Super Boiler
• Super Critical CO2-based Power Generation Technology

Let’s hope our engineers bring us to these new-techs earliest, and we in near-future may see a better, efficient and affordable waste heat recovery systems.

I would like to thank my group members Harish Chaudhari, Maitreyee Chaudhari, Rahul Biradar and Rutuja Chandanshive for their immense contribution in successfully detailing this blog.

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