SIEMENS

Steel is everywhere. Without this material entire industries, such as mechanical engineering and auto manufacturing, would be inconceivable. In 2011, worldwide production of crude steel amounted to about 1.5 billion metric tons — a figure that required a vast amount of energy to achieve. Even steel produced by recycling scrap metal in electric-arc furnaces, for instance, requires approximately 370 kilowatt hours (kWh) per metric ton. In this type of furnace an electric arc is struck between several electrodes. The resulting heat causes the steel to melt. And then? The furnace’s tap hole emits a mixture of gases at up to 1,700 degrees Celsius — a huge waste that might otherwise be used to generate electric power.

Capturing that waste heat is exactly what a team headed by Dr. Alexander Fleischanderl of Siemens VAI Metals Technologies in Linz, Austria is working on. But there’s nothing easy about it. From furnace loading, through the melting process, to “tapping” takes between 45 and 60 minutes, depending on the system. During that period, the flue gas temperature and the flow rate vary widely, which poses a difficult challenge for engineers. “To operate effectively, a turbine must be fed hot vapor continuously,” Fleischanderl points out. “To make that happen, we have introduced a heat storage system between the furnace and the turbine.”

This function is provided by a salt mixture with a low melting point that is also used in solar thermal power plants. The salt mixture extracts energy from the exhaust gas through a system of heat exchangers. In a second circuit, water flows through the heated salt mixture, generating steam that powers a turbine that generates electric power. The latter is a continuous process that is independent of the furnace cycle.

“Salt melts have the advantage that they require no pressure, have high storage capacity, and are environmentally harmless,” explains Fleischanderl. Consequently the system functions without expensive high-pressure vessels, is easier to build and to get approved, and is safe to operate. What’s more, due to the salt’s high temperature of up to 500 degrees Celsius, the process enjoys an efficiency rating of 24 percent — substantially higher than that of a steam accumulator, which is only 17 percent.

Fleischanderl estimates a worldwide market of around 300 such systems, each of which could cost about €30 million. “Up to 20 percent of the electric power needed for melting scrap metal could be recovered from the waste heat,” he figures. “That would reduce CO₂ emissions per metric ton of steel by about 40 kilograms (kg). Current systems emit about 270 kg of CO₂, 220 kg of which results from power generation. This means that CO₂ emissions from a typical 120-metric-ton furnace could be reduced by more than 30,000 metric tons annually.”

Experts from Siemens are currently testing different vessel materials and salt mixtures, and a pilot system has been in operation since February 2012 in a steel plant in Germany. The first commercial heat recovery system may be available in 2013.

But at present, most industrial waste heat still serves no useful purpose. “Today about half of the primary energy consumed in industrial processes and in energy generation goes to waste,” says Dr. Martin Tackenberg of Siemens Corporate Technology (CT) in Erlangen. “Especially when it comes to waste heat at temperatures below 300 degrees Celsius, there are hardly any economically practical and technically mature processes.” With this in mind, Siemens, in a special project that focuses on thermal management, has examined a wide range of processes and identified 20 cases that could offer substantial heat recovery potential. Drawn from about 80 use cases, the most promising projects include four in which Organic Rankine Cycle technology (ORC) can be applied. ORC is particularly well suited for using waste heat from furnaces in the glass industry, from diesel or gasoline engines, from gas flaring at refineries, and from gas turbines in compressor stations. In contrast to the classic steam-circuit process, what circulates in an ORC is not water but an organic medium that ensures optimal efficiency at low waste-heat temperatures and low power, and is well suited for a compact design.

A Volatile Medium. In an ORC research project with the Moscow Power Engineering Institute and Moscow State University, Siemens is using a new working medium from U.S. company 3M, which is composed of carbon, fluorine, and oxygen and vaporizes at just 49 degrees Celsius under normal pressure. “What’s more, this organic medium is absolutely non-polluting. That’s very important to Siemens in the context of sustainability,” notes Tackenberg. The first demonstrator, which has a power output of 1.2 kilowatts (kW), has been operating at Moscow State University since November 2011. A scaled-up model with a power output of 100 kW is slated to start operating at German fiberglass manufacturer Lauscha’s Russian plant in the fall of 2012. It will derive its energy from hot waste gas produced by a fiberglass production line — at a temperature of only 220 degrees Celsius. “This ORC unit will produce about 800,000 kWh of extra electricity per year with an efficiency of about 20 percent, which corresponds to a value of about €80,000,” says Tackenberg. “The investment cost in this case is around €2,200 per kilowatt. As a result, the system will be amortized in less than three years.” He estimates the annual market for ORC solutions amounts to about €3 billion.

Cleaning Water with Waste Heat. Not only can waste heat be used to produce electricity. It can also be used to clean water. That’s the idea behind a Siemens process called EvaCon (Evaporation and Condensation). “Industrial processes often generate waste heat at a temperature that’s too low for economical electricity generation,” explains Dr. Thomas Hammer, project manager of EvaCon at CT in Erlangen. “EvaCon can utilize waste heat at between 65 and 90 degrees Celsius for water purification.”

In this application, wastewater is heated, evaporated and fed into a condenser, where the vapor liquefies again. “This is how pure water is separated from concentrated wastewater, and the end result is demineralized water,” says Hammer. Potential heat sources include, among others, paper mills and soft drink bottling plants where wastewater is generated that cannot be readily disposed of in a sewage treatment plant. EvaCon generates new freshwater while at the same time reducing the amount of wastewater that must be disposed of.

CT researchers are currently investigating the best design and materials for the vaporizer and condenser. In September 2012 a prototype is slated to demonstrate that the process can be used on an industrial scale. Tackenberg estimates that EvaCon will be market-ready by 2015 — and that by then it will be an extremely appealing product. “On an annual basis, if a soft drink producer needs eight cubic meters of sterile rinse water per hour and disposal of associated wastewater, it currently has to pay about half a million euros per year per bottling line,” he explains. “The use of EvaCon for the reprocessing of wastewater saves about €370,000 annually. So, given a cost of €325,000 to build the system, the investment would be amortized in less than a year.” CT is working with Siemens Industry Sector’s Food & Beverage Unit to promote EvaCon, and has already presented the process to Pepsi in New York.

Wellness for Machines. In another project, Siemens researchers are investigating how latent-heat storage units could ensure the “thermal well-being” of tomorrow’s machines. In such storage devices, heat does not cause an increase in temperature but instead a phase transition — for example, ice melting to form water — which remains at zero degrees Celsius until even the last bits of ice have melted. High-precision machine tools in particular require complex thermal management. During operation they are cooled, and prior to start-up they are heated so as to prevent an increase in rejects that would result from thermal expansion of the tools and of the products. Latent-heat storage units can absorb excess energy during production and release it to the machine again during periods of inactivity. “That would substantially reduce cooling costs, and no additional energy would be required for the start-up,” notes Tackenberg.

This project is still in the starting phase, so no one knows yet which material will be best for latent heat storage. ORC and EvaCon solutions from Siemens are therefore likely to see practical use before latent-heat storage units do. Tackenberg envisions that by 2020 these and other developments could result in a reduction of fossil primary energy escaping as waste heat from industrial smokestacks from levels as high as 50 percent to just around 40 percent.

According to a study conducted by Siemens and McKinsey, between 1.1 and 2.5 gigawatts of usable waste-heat power could be harvested from ORC technology applications alone. This would benefit not only the climate but also suppliers of such solutions. Siemens will definitely be one of them.