Rankine Cycle and Organic Rankine Cycle
Waste heat is a vast, underutilized, and sustainable source of energy that is often overlooked due to the costs involved with capturing and converting that heat into electricity. Rankine cycle (RC) systems have been used for decades to generate clean electricity from heat, however these systems are restricted to sources of high-temperature heat such as found at large power plants, industrial complexes, and large geothermal sources.
With the development of Organic Rankine Cycle (ORC) technology, a refinement of the Rankine Cycle, there is now the ability to capture and convert lower-temperature heat into electricity. The main difference between the two cycles is that RC’s use steam to produce power while ORC’s convert an organic working fluid, which has a boiling point less than that of water, into vapor to produce power. This differentiating factor makes for much smaller systems, allowing lower temperature heat sources – such as engine waste heat and process heat – be used to generate emission-free electricity.
Screw Expander vs. Turbine Technologies
However, not all ORC systems are created equal. There are those that use a turbine as their power generator whilst others use a screw expander. There are advantages and disadvantages of each system which will be briefly discussed here, however, it is important to note that net-zero cooling to power technology is a solution predominantly utilizing screw expander technology.
Turbine systems are generally suitable for stable heat streams with constant load. They offer a slight increase in efficiency over expanders due to less friction loss and extremely high operating speeds. However, with blades spinning at such high speeds, turbine systems are at increased risk of being damaged by liquid impingement, or the formation of liquid drops in the pressurized vapor. As a result, this cycle requires superheated vapor throughout the process to avoid condensation, which may occur due to a drop in pressure. If this occurs the blades could be badly damaged and must be replaced. This creates an issue with smaller, more variable heat sources. If the turbine cannot maintain superheat inlet parameters, the system will have to shut down or go on bypass to avoid damage.
As indicated above, while a turbine system may be ideal for higher temperature heat sources with a consistent flow, they tend to be unsuitable for many waste heat recovery opportunities that screw expander systems are able to capitalize on due to their superior transient operation capabilities.
Screw expanders are more compact, cost-efficient, and robust compared to turbine systems. Their design results in lower operating speeds, quieter operation, and less maintenance. Since the system is operating at much lower speeds, it experiences less pressure on the pump leading to reduced maintenance costs and easier operation. The robustness of an expander allows it to tolerate “wet” dual-phase flow caused by incomplete phase change of the working fluid. This makes screw expanders better equipped for transient operation, allowing the system to reliably generate power from heat sources with fluctuations in thermal input, both temperature, and flow. Additionally, expander systems have a far superior turndown ratio, with leading systems able to operate everywhere from 5 kW up to 125 kW.
ORC systems incorporating a twin-screw expander are not only more affordable and reliable but are more suited for low temperature waste heat recovery where heat availability is variable and are particularly well suited for net-zero cooling applications.
The Potential of Waste Heat
Power generation and industrial processes, amongst other sectors, lose a great deal of their applied energy as heat – which in many cases can be in excess of 50%. This lost energy is commonly known as waste heat. For internal combustion engines, waste heat is rejected through the engine jacket water system and exhaust gases. With that said, there are many different sectors with different forms of waste heat. Providing the waste heat can be harvested and converted into a fluid heat source through the use of heat exchangers, an ORC system can be integrated to recycle that heat into clean, green electricity.
The heat produced as a byproduct in these processes can either be friend or foe, depending on how one approaches the situation. Take combustion engines (or gensets) for example. Engines see extensive use around the world, from remote power generation, marine propulsion, and anaerobic digestion to LFG production and gas compression. Regardless of their use, engines generate substantial amounts of heat. This heat is a waste of thermal energy and steals valuable power from the engine for cooling. Waste heat and the parasitic cooling load amount to a large portion of the energy lost in power generation processes.
Modern ORC systems can convert heat sources as low as 70°C into clean power. These new systems have undergone many years of development to ensure they are more cost-efficient, robust, and reliable than ORC systems of the past. Today’s sophisticated, yet simple, technology has become a reoccurring area of interest when it comes to energy efficiency and sustainability. The potential for heat recovery from low temperature sources opens opportunities for businesses both small and large to take advantage of their waste heat to achieve greater energy efficiencies and improve their bottom line while taking steps to better the planet.
What is “Cooling To Power”?
Cooling to power refers to an ORC units’ ability to act as a combined cooling and power generator, providing a net-zero cooling solution that generates power as a secondary function when the cooling load is not at peak demand. Cooling to power is an effective means of increasing energy efficiency due to ORC systems consuming the waste heat as fuel, which significantly reduces the cooling load (70-100%). This means in addition to generating emission-free power, the parasitic cooling load also is reduced or even eliminated, further reducing costs and increasing efficiency.
As the heat load is reduced, this allows cooling to power systems to prioritize the generation of clean electricity. When the cooling load increases the system will automatically adjust power output to fulfill engine cooling requirements, maintaining engine cooling independent of the generator’s operating status. In rare occurrences, during peak demand, the ORC expander is bypassed completely and the system will prioritize full-load cooling. This is the only time the system will consume power that is not its own.
Essentially, cooling to power technology provides a highly efficient, self-powered radiator that pays for itself through the generation of power. While traditional cooling systems (radiators / cooling towers) consume power to provide cooling, cooling to power systems consume heat to provide cooling as well as power. This provides a cooling solution with a positive net present value (NPV) opposed to a negative NPV.
Quick Numbers
If electricity rates in an area are $0.10/kWh and a radiator experiences 8,000 hours of operation annually while consuming 8 kW – that amounts to 64 MWh annually valued at $6,400. If the radiator were replaced with a cooling to power system, such as ElectraTherm’s Active Cooler, and produced an average of 40 kW while displacing the previous demand of 8 kW – that is 48 kW of newly available electricity, or 384 MWh annually valued at $38,400. Over a 20-year period, all variables consistent, the Active Cooler would boost revenue by $768,000 while a standard radiator would cost $128,000.
Previous case studies that have demonstrated the ability of cooling to power technology have seen efficiency increases in the 5% range – a small number that makes a big impact on businesses’ bottom line and the environment.
Economic and Environmental Implications
Increasing energy efficiency is the single easiest step in pursuing a carbon-neutral future. Using an existing resource – heat – to provide power and cooling reduces fossil fuel consumption and reliance on the grid. ORC power generation is a sustainable technology that reduces the amount of energy consumed (fuel) and energy wasted (heat). Energy efficiency will always play a role in combatting climate change because no matter the process, the more efficient it is, the less impact it has on the planet.
With the reliability and cost-effectiveness of modern systems, waste heat recovery solutions such as cooling to power are both profitable and practical. With the world eyeing ways to shift toward clean energy and with the help of incentives promoting sustainability like the Consolidated Appropriations Act 2021, cooling to power is primed to be a disruptive technology in the energy efficiency and commercial cooling market – effectively changing the way corporations see and use waste heat.
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About ElectraTherm
ElectraTherm by BITZER Group provides world-class waste heat recovery solutions by utilizing the Organic Rankine Cycle along with proprietary technologies to convert low-temperature heat into clean power. ElectraTherm’s simple and effective solutions generate clean electricity, boost efficiency, reduce energy costs, and decrease emissions – with no additional fuel consumption. Having shipped over 100 ORC units to over 13 countries clocking over 2,000,000 hours of operations, ElectraTherm is a global leader in small-scale waste heat recovery.
The combined advantage of ElectraTherm’s engineering along with the value of being supported by BITZER, the world’s largest independent manufacturer of refrigeration compressors which boasts close to 3,500 global employees and average annual sales approaching $1 billion, allows the ElectraTherm team to continue developing industry-leading ORC technology that is good for both business and the planet.
ElectraTherm’s Power+ Generator is a heat to power solution, converting waste heat into clean electricity and usable thermal. The Active Cooler serves as a net-zero cooling to power solution, using the same ORC process to provide self-powered cooling while generating electricity. Both of ElectraTherm’s solutions reduce emissions and energy costs while providing a baseload power / cooling supply. Taking advantage of waste heat is not only profitable, but practical. Allowing companies to benefit from a more circular economy while achieving sustainability goals.