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Industrial Waste Heat — Brief Summary

One third of the world's energy is used by industry to manufacture everything from fertilizer and foodstuffs to steel and cement.

To improve energy efficiency, lower costs, and reduce emissions, most industrials implement ways to recover and reuse otherwise wasted thermal energy. The most common approach leverages solutions like cleverly arranged heat exchangers that transfer heat directly from hot process streams that need cooling (such as exhaust gases and cooling water) to streams that require heating. In many cases, excess heat is furthermore collected and exported to a nearby district heating network, greenhouses, or an adjacent industrial process.

Despite these efforts, a substantial portion (20-50% depending on sector/process) of the energy is discarded as ultra-low-grade waste heat (lower than ~60°C). This low-temperature heat is difficult to reuse because most industrial processes require steam or higher-grade heat (130-170°C or above). As a result, many plants are forced to shed this thermal energy to the atmosphere via radiators or simply pour it down the drain as warm cooling water.

Currently Available Systems

An established category of solutions for upgrading such low-grade waste heat into usable process steam is the integration of Large Heat Pumps (LHP) with Mechanical Vapor Recompression (MVR). These combined "LHP + MVR" system packages are offered by established OEMs and technology providers, including Turboden (a Mitsubishi Heavy Industries group company), GIG Karasek, Piller Blowers & Compressors, Atlas Copco, and others.

A typical LHP + MVR system consists of three steps:

1. A closed-cycle large heat pump first upgrades the available low-grade waste heat. It does so by concurrently cooling a ~40°C waste-heat source, thus extracting its thermal energy which it uses to heat up thermal fluid to a higher intermediate temperature level, 70-90°C, or more in the case of high-temperature heat pumps (HTHPs). It's similar to how refrigerators work, cooling the food inside by moving the "inside-heat" energy to the hot coils that used to be visible on the backside of refrigerators.
2. The intermediate heat from the thermal fluid is transferred to an evaporator vessel (often referred to as a vacuum flash tank). The tank is operated at sub-atmospheric pressure which reduces the boiling point of the water in it. Due to the reduced pressure the 80°C thermal fluid from the heat pump is sufficiently hot to boil the water in the evaporator vessel and generate low-pressure steam or vapor from the water (or process condensate).
3. One or more MVR compressors then extract and compress the low-pressure vapor. The rapid and powerful compression simultaneously increases the steam's pressure and temperature, quickly turning it into useful saturated steam suitable for direct process use, typically in the range of 130-170°C or higher, depending on the number of compression stages and system design.

The LHP + MVR approach enables significant temperature lifts from ultra-low-grade sources while producing steam that significantly supplements the steam already generated for existing plant steam networks. Generating process steam this way can be far more cost-effective than producing the same amount in on-site gas boilers. An important caveat is the fact that electrical energy is typically 3-5x more expensive (per Joule) than natural gas. Because of this unfortunate price discrepancy, the efficiency (COP) of electrically driven waste-heat upgrading equipment is of paramount importance.

The design and specifics of LHP + MVR systems and high-temperature heat pumps vary between manufacturers, and size ranges from a couple of kWth to several MWth. The technology is now considered mature and has been successfully integrated in thermally intensive industries such as pulp & paper, food/beverages and chemicals.

Other technical solutions are less mature or have attributes making them less comparable to Hydram's solution (such as ORCs, AHTs, THTs, TVRs, thermoelectrics and others).

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