YANMAR Technical Review

Abstract

In order to enable the reduction of CO₂ emission, Yanmar has been developing power generation systems that uses exhaust heat generated from various industries. Yanmar E-Stir Co., Ltd. focuses on technology of generating electricity from unused heat energy, developing Stirling engines that will contribute to the reduction of CO₂ emissions. The potential of unused heat energy in Japan will be described, and waste heat power generation technologies of Yanmar E-Stir and future expectation will also be discussed.

1.Introduction

Achieving carbon neutrality requires the early social implementation of CO₂ emissions reduction and energy-saving technologies. Effective utilization of unused thermal energy is necessary to promote energy efficiency⁽¹⁾. In Japan, approximately 60 % of primary energy goes unused in the form of waste heat, and most of this untapped energy is small-scale and dispersed, thus making it difficult to utilize. However, using Stirling engines to convert this energy into electric power represents one way to overcome this challenge, and Yanmar E-Stir Co., Ltd. is currently developing technology to do this. Deploying these Stirling engine power-generation systems in practice and expanding their scope of application will require finding ways to operate them with new high-temperature heat sources or in environments that are highly corrosive and harsh on equipment. This article explains the underlying theory and principles of operation of Stirling engines, describes the power-generation systems Yanmar E-Stir is developing, and what can be expected from them in the future.

2.Stirling Engine

2.1.Main Features

A Stirling engine is a type of external combustion engine that draws thermal energy from external sources. As indicated in Table 1, standard gasoline engines derive motive power from the expansion of combustion gases created by burning gasoline inside the engine. In contrast, the working gas in a Stirling engine expands by thermal energy from external sources obtained via a heater (heat exchanger). While the output of Stirling engines is lower than gasoline engines of the same size because of their inability to produce a high compression ratio, they benefit from being able to generate motive power from a wide range of external heat sources. This means that Stirling engines can be paired with generators to convert unused thermal energy to electric power. And, while there are other techniques for accomplishing that conversion, Stirling engines have the advantage of being comparatively easier to downsize and so more suitable for use with small-scale heat sources because they do not require an evaporator that generates steam and a turbine. For these reasons, Yanmar E-Stir believes that Stirling engines are an effective technology for use with small-scale and dispersed sources of unused thermal energy found in the industry.

Table 1 Comparison of Gasoline and Stirling Engines

2.2.Configuration

Fig. 1 shows a schematic diagram of Stirling engine. The engine cylinder contains separate displacer piston and power piston that are each connected to the crankshaft with a 90° phase difference between them.
The engine’s working gas is permanently contained within the cylinder. The displacer piston separates the hot workspace (where the working gas is at a high temperature) from the cold workspace (where the working gas is at a low temperature). The engine also provides a channel for the working gas to move between the hot and cold workspaces, causing the gas to be heated or cooled by the heater, regenerator, and cooler as it moves between these workspaces.
The changing relative sizes of the hot and cold workspaces as the displacer piston moves through a full cycle forces the working gas through this channel, heating or cooling the gas and thereby causing it to expand or contract.
The power piston is driven by the working gas, as it moves through a full cycle, it drives the crankshaft and thereby generates engine output.

2.3.Principles of Operation

Fig. 2 shows the principles of operating a Stirling engine. As the figure shows, the engine operates on a four-stroke cycle: (1) Heating, (2) Expansion, (3) Cooling, and (4) Compression. As it operates, the external heat supplied to the engine is converted into rotational torque in the crankshaft and this generates the engine’s output.

2.4.Thermal Efficiency

In ideal terms, the Stirling engine cycles through the four states of isothermal expansion, isovolumetric cooling, isothermal compression, and isovolumetric heating. The theoretical thermal efficiency of this cycle is equal to the Carnot efficiency, which is expressed in terms of the ratio between the low and high temperatures ( and ). Carnot efficiency is the efficiency of the Carnot cycle, which has the highest thermal efficiency of any heat engine. As shown in equation (1), the thermal efficiency of a practical Stirling engine () is expressed in terms of the Carnot efficiency multiplied by a Carnot efficiency coefficient (). This Carnot efficiency coefficient () is the ratio of the Stirling engine’s thermal efficiency () to the Carnot efficiency.

As indicated by equation (1), the smaller the value of the low to high temperature ratio () is, the greater the efficiency () is achieved. Accordingly, in order to convert heat into work with high efficiency, the temperature () of the heat source used to drive the Stirling engine needs to be as high as possible. However, this temperature is constrained by the high-temperature strength and corrosion resistivity of the materials used in the engine’s heater and other hot components.

3.Stirling Engine Power Generation System

3.1.Main Specifications

Table 2 lists the main specifications of the Stirling engine power generation system being developed by Yanmar E-Stir. The engine has a maximum power output of 9.9 kW, work with heat sources between 500 °C and 800 °C, and supply a three-phase AC200-V output to the electrical grid. The engine system includes a controller that monitors the heat source and automatically controls the level of power generation accordingly, starting, operating, or shutting down the generator as needed (see Fig. 3).

Table 2 Main Specifications⁽²⁾

As presented in the Winter 2017 issue⁽³⁾ of YANMAR Technical Review, a variety of heaters can be used to collect the waste heat depending on the corrosiveness of the heat source. The following explanation uses a heat source with a less corrosive exhaust gas and an incinerator with a highly corrosive exhaust gas as examples.
The upper row of Table 3 shows a heater used in an industrial furnace that discharges a less corrosive exhaust gas and which works on the basis of direct contact between the heater and high-temperature exhaust gas. This form of heat transfer involves optimizing the length and number of heater pipes to obtain the required heat transfer surface area without reducing the mean flow rate of the exhaust gas that passes over the outer surfaces of the heater pipes. Moreover, when retrofitting a heater onto an existing flue duct because there is often little leeway for pressure loss in the flue, the heater needs to be installed at a location and in such a way that does not result in additional power use by exhaust fans or other equipment.
The bottom row of Table 3 shows a heater used in an incinerator that discharges a highly corrosive exhaust gas. To avoid problems with heater pipe corrosion or clogging, the heater pipes are encased in a protective cover. In this case, the transfer of heat to the heater pipes occurs by means of radiation from the protective cover. The effective heat received from radiated energy is governed by the Stefan-Boltzmann law and is proportional to the difference between the fourth power of the protective cover's inner surface temperature and the fourth power of the heater pipes' outer surface temperature, so this method works best when the waste heat is at a high temperature.

3.2.Example Installation

The first example describes the heater of a Stirling engine installed in an industrial incinerator to recover heat from its less corrosive exhaust gas. Combining the industrial furnace with a Stirling engine makes it possible to create an industrial furnace that can use waste heat to generate power. Fig. 4 shows an example of installation in an industrial batch furnace for firing ceramic products. In a batch furnace, the product is placed in a basket for heat treatment. When the heat treatment is finished, the lid is opened to remove the product. Every time the product is subjected to heat treatment, it is placed inside the furnace and taken out again, resulting in significant temperature fluctuations in both the furnace and the exhaust gas. The developed Stirling engine maintains power generation throughout a wide temperature range and automatically adjusts the engine rpm to a value suited for the exhaust gas temperature. This allows for longer power generation time even with heat sources with significant temperature fluctuations, leading to an increase in the amount of electricity recovered.

The next example describes the heater of a Stirling engine that is installed inside the flue of the incinerator at a general waste disposal facility to recover heat from the hot, highly corrosive exhaust gas. Fig. 5 and Fig. 6 show a conventional incinerator used by local governments to burn waste collected from homes and businesses at temperatures between 800 °C and 1000 °C, with an exhaust gas containing corrosive dust. The Stirling engine's heater is fitted with a cylindrical protective cover to protect it from the hot, highly corrosive exhaust gas. Some of the thermal energy from the exhaust gas is transferred to the protective cover and then radiated from the inner surface of the cover to heat the outer surface of the heater pipes. The Stirling engine can be installed on the side wall of the incinerator by inserting the heater using holes provided for this purpose in the refractory bricks of the side wall. In this way, it is possible to install multiple Stirling engines in the empty spaces around the high-temperature parts of the incinerator, including the furnace side walls, ceiling, and flue.

3.3.Environmental Performance

The Stirling engine power generation system can reduce both electricity purchases and CO₂ emissions. A single Stirling engine can reduce an amount equivalent to 34 t of CO₂ emissions annually when converted using the Power Company A's CO₂ emission intensity coefficient.

4.Future Outlook

As described in this article, Yanmar E-Stir supplies customers with a complete system in which the Stirling engine is paired with a generator. Installation is simple and does not require a lot of space, with the heater unit only needing to be in direct contact with a heat source to operate the engine. The system can be used with a variety of different forms of heat transfer (convective, radiative, and conductive) and has already been deployed in a number of different industries ⁽⁴⁾⁽⁵⁾. However, the current practice for most high-temperature exhaust gas from industrial sites is to cool it with air at ambient temperature before being discharged into the atmosphere at a low temperature (A-(1) and B-(1) in Table 4). Continuous furnaces like that in Table 4 B are used as kilns for firing tiles or other ceramic products, which are conveyed through the furnace on a belt conveyor or similar. Meanwhile, as explained above, Stirling engines achieve greater efficiency at the higher temperature of the heat source. This means that a method of cooling exhaust gas or products that not only lowers their temperature, but also converts the heat into electric power is required. Installing Stirling engines at locations in a factory where high-temperature exhaust gas is present so as to supply this heat to the engine (the A-(2) and B-(2) examples in Table 4) can achieve this. Moreover, if Stirling engines are to be adopted more widely, further progress is needed to enable their use with new high-temperature heat sources or in harsh environments where installation is not currently viable. Accordingly, Yanmar E-Stir intends to work on powering Stirling engines with the heat from high-temperature materials such as the furnace itself (C-(2) in Table 4). To enable use with sources of heat that also contain dust or corrosive substances, Yanmar E-Stir is also investigating designs that will provide the protection of the heater unit in contact with the heat source from contamination (D-(3) and E-(3) in Table 4).

5.Conclusions

This article has explained the features and principles of operation of a Stirling engine when used as a means of converting waste thermal energy into electric power and described the work being done in this field by Yanmar E-Stir Co. Ltd. Through its research and development of Stirling engines, Yanmar E-Stir is providing a way to make use of waste thermal energy in industry. This currently includes work on operating the engines with new high-temperature heat sources and in environments that are highly corrosive and harsh on equipment with the aim of expanding their scope of application and encouraging their wider use. In the future, Yanmar E-Stir intends to continue its work on reducing CO₂ emissions and helping society achieve decarbonization.

References

• (1)NEDO, Research and Development Project on Innovative Thermal Management Materials and Technologies, May 13, 2022
  https://www.nedo.go.jp/english/activities/activities_ZZJP_100097.html
• (2)Yanmar E-Stir, Catalogue (Exhaust Heat Recovery Stirling Engine)
  https://www.yanmar.com/jp/about/company/e-stir/business.html
• (3)Kitazaki, et al., Development of Zero Emission Power Generating System “Stirling Engine”
  YANMAR Technical Review, January 27, 2017
  https://www.yanmar.com/global/about/technology/technical_review/2017/0127_5.html
• (4)Akazawa, et al., Stirling Engines, Their Trend as Electric Power Generators from Waste Heat, The Journal of The Institute of Electrical Engineers of Japan 136(9) 2016, pp. 601-607 in Japanese.
  https://doi.org/10.1541/ieejjournal.136.601
• (5)Akazawa, Stirling Engine Power Generation System for Utilizing Waste Heat, and its Trial Application: Utilization of Waste Heat, Marine Engineering: Journal of the Japan Institute of Marine Engineering, Vol. 51, No. 1 (2016), pp. 102-109 in Japanese.
  https://doi.org/10.5988/jime.51.102
• (6)Emission Factor by Electric Utility Operator, Ministry of the Environment and Ministry of Economy, Trade and Industry, 2022 (website in Japanese)
  https://ghg-santeikohyo.env.go.jp/calc/

-IMPORTANT-

The original technical report is written in Japanese.

This document was translated by Innovation & Technology Division, Technology Strategy Division.