Industry sectors and business owners are continuously looking for ways to drive costs down by reducing their energy consumption to be more competitive. The process industries traditionally are some of the heaviest energy-consumption sectors. To make matters worse, it is estimated that somewhere between 20 percent and 50 percent of the energy used by the industries overall is lost as waste heat. As the industrial sector continues efforts to improve its energy efficiency, recovering waste heat losses provides an opportunity for an emission-free and less costly energy resource.
The organic rankine cycle (ORC), among other waste heat recovery technologies, has been studied extensively in the past decade or so for practical and economical use in recovering industrial waste heat. It has gained attention recently based on its potential to recover the abundant low grade — normally medium-to-low temperature — waste heat available in the process industries, among other industry sectors.
Where Is Heat Wasted?
A 2010 Frost and Sullivan study states that in the United States’ pulp and paper industry alone, more than 332x1012 BTU/year of heat is wasted, based on 2008 production data. The major processes where heat is wasted are cooking/digesting, pulp drying, stock preparation and pressing, and paper drying. The total amount of heat wasted is equivalent to more than 12 GW of electricity.
In the food processing industry, waste heat from rotary steam-tube dryers, flash dryers, steam pasteurization, ovens and evaporators easily adds up to more than 250x1012 BTU/year (more than 9 GW of electricity) energy loss, according to a 2004 U.S. Department of Energy report.
Process industries such as chemicals and petrochemicals, food processing, pharmaceuticals, plastics and rubber processing use various heating equipment to process materials. A large amount of the heat is wasted in the form of exhaust or flue gas out of this heating equipment.
But waste heat is not only from exhaust gas. It can actually come in different forms:
- Flue gas.
- Hot water or other hot liquid.
- Radiation heat.
It can be exhaust or flue gas from a boiler, cooling water, or surface and radiant heat from hot equipment such as ovens, melters, heaters, dryers, furnaces and burners. The temperature range spans from several hundred degrees to more than 1000°F (538°C). For example, a deep fryer used by a food manufacturing facility typically has a frying temperature ranging between 300 and 375°F (149 to 191°C). The exhaust gas from the fryer could easily reach 100 to 200°F (38 to 93°C).
In most cases, lower temperature waste heat — especially when the temperature is lower than 700°F (371°C) — can be recovered effectively and efficiently using ORC technology.
How does the Organic Rankine Cycle Work?
The ORC uses an organic, high molecular mass fluid with a boiling point occurring at a lower temperature than the water-steam phase change (figure 1). The organic fluid allows heat to be recovered from lower temperature heat sources than heat used by conventional water-steam cycles. The low temperature heat then is converted into useful work (typically through an expander), which is further converted into electricity by a generator.
A typical ORC generating system consists of an evaporator, a main ORC expander and a generator, a condenser and additional auxiliary equipment. In operation, the heat source transfers thermal energy into the ORC system to generate electrical energy. The evaporator vaporizes and pressurizes the organic working fluid. The pressurized organic fluid enters an expander, where expansion of the fluid drives a turbine to generate electrical power. Following expansion, the working fluid then is condensed through a condenser back to the liquid phase and is further fed back into the system, through a reservoir and a pump, to repeat the closed-loop cycle.
What to Consider When Choosing an Organic Rankine Cycle
When it comes to any project for waste heat recovery, it is important to first identify the heat source. Steam and hot liquid normally are the most preferred heat source forms for an ORC. It should be pointed out that not all processes are candidates for waste heat recovery using an ORC. Recovery from exhaust gases may be possible, but volumes and temperatures may be too low to provide financial justification. In some cases such as with radiation heat, the heat is almost impossible to recover economically. Additionally, flue gases should be fairly clean and free of contaminants such as corrosive gases and particulates. Otherwise, it is recommended that an intermediate hot-water or thermal-oil loop be added to reduce the risk of heat exchanger erosion. This addition could incur additional costs.
To summarize, several parameters must be evaluated at the beginning of a waste heat recovery project:
- The flow rate of the exhaust gas or other heat source forms.
- The temperature of the exhaust gas or other heat source forms.
- The composition of the exhaust gas or other heat source forms.
All these factors determine the total available thermal energy and are used to select optimal heat exchangers.
After identifying usable heat sources based upon the available heat source temperature and flow rate, the next step is to select an ORC technology vendor. Here are some essential factors to consider during the vendor selection process:
- Proven technology.
- ORC system overall efficiency.
- System maintainability and total operating and maintenance costs.
- Auxiliary equipment selection and associated costs.
- Project lifecycle cost.
- System reliability and availability.
The other important aspect for any waste heat recovery project is to determine the accepted financial payback period. The economic potential of an ORC waste heat recovery system depends on the capital recovery, which in turn, depends on the annual energy (normally fuel or electricity) savings. Some facilities use the power generated by an ORC system internally in their manufacturing process to offset electricity costs. Some sell the electricity back to utility grid. Some do both. The final usage of the power determines whether the benefit or savings will be realized within the financial payback expectation, and it further dictates the total project budget.
An example can help show how to determine the ORC project’s financial payback time. Assume a food manufacturing facility uses an oven for its drying process. The oven emits about 700°F exhaust gas at around 40,000 pounds per hour. In this case, assume the right ORC system is installed to recover the waste exhaust gas, which makes about 100 kW electricity. The total installed ORC system cost is assumed to be $350,000. The facility pays $0.15 per kilowatt-hour for its electricity usage. The power generated by the ORC is consumed onsite in the manufacturing process.
Assuming 8,000 total operation hours per year, the annual electricity saving by installing the ORC system is calculated to be:
100 kW × $0.15/kWh × 8,000 hr/yr = $120,000
The return on investment in this particular case is less than three years, given the assumed system cost of $350,000, divided by $120,000 per year savings, which equals 2.92 years.
In addition to the tangible benefits like fuel and energy savings, it is important to consider the intangible benefits and savings of ORC waste heat recovery. These include emission reduction, carbon footprint reduction and energy efficiency improvement.
In summary, a substantial amount of waste heat could be recovered by using ORC technology for process industry applications. It is important to first evaluate the heat source and then determine the optimal ORC technology. The most appropriate type of ORC equipment is determined based on technical feasibility, annual cost savings and project capital cost. The tangible and intangible benefits of an ORC system will help businesses reduce costs, boost their bottom line and stay ahead of the competition.