Thermoelectric topping cycles for power plants to eliminate cooling water consumption

doi:10.1016/j.enconman.2014.04.031

Energy Conversion and Management

Volume 84, August 2014, Pages 244-252

https://doi.org/10.1016/j.enconman.2014.04.031 Get rights and content

Highlights

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Complete system analysis of a thermoelectric topping generator in a power plant.
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Topping application does not require high-ZT thermoelectrics to be effective.
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The improved efficiency can be used to replace water cooling with air cooling.
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The topping generator is superior to flue gas waste heat recovery in efficiency and less materials.

Abstract

This work shows that thermoelectric (TE) topping generators can add 4–6% to the overall system efficiency for advanced supercritical steam turbines (Rankine cycle) that nominally generate power with 40–42% efficiency. The analysis then considers how this incremental topping energy can replace cooling water flow with air-cooled condensers (ACC) while maintaining current power output and plant efficiency levels with commensurate economic benefit ($/kW h). The simulated TE modules are located inside a coal-fired boiler wall constructed of wet steam tubes. The topping TE generator employs non-toxic and readily available materials with a realistic figure-of-merit range (ZT = 0.5–1.0). Detailed heat transfer and thermal analyses are included for this high-temperature TE application (e.g., 800 K for the cold side reservoir). With the tube surface enhanced by fins, the TE elements are designed to perform optimally through a distributed configuration along the wall-embedded steam tubes that are more than 20 m high. The distribution of the gas temperature in the furnace along the wall height is predicted by thermo-fluid dynamic analysis. This foundational design and analysis study produces overall realistic efficiency predictions in accordance with temperature–entropy analysis for superheated Rankine cycles. Lastly, the approach also allows for the addition of waste heat recovery from the flue gas. The analysis shows that the power output from the topping TE generator is significantly larger, compared to that from the waste heat recovery, due to the larger available temperature difference.

Graphical abstract

Introduction

Improved energy efficiency of power production is still important for most common coal-fired power plants, which provide 50.4% of electricity supply in the U.S. [1] while the penetration of renewable energy sources remains hindered by capital cost, intermittency, and seasonal swings [2], [3]. Some large solar concentrated power plants using Rankine cycles operate primarily in desert areas [4]. Energy efficiency is not only important for economic reasons, but it is also critical for conserving natural resources [5]. We investigate the performance and economic impact of adding thermoelectric (TE) topping generators to provide additional power output from current-technology coal-fired boiler furnaces within an advanced supercritical steam turbine (Rankine cycle).

Fig. 1 shows results from a prior analysis for a combined cycle [6], which indicates an optimum steam temperature for maximizing total output power. This additional power output can cover the deficit in power output by higher temperature condensation utilizing an air-cooled condenser [7]. There is another way to enhance the total power output by utilizing a waste heat recovery cycle either by thermoelectric or other energy conversion principle. The waste heat recovery can coexist with the topping cycle without mutual interference. Waste heat recovery TE generators for automotive exhaust applications [8], [9] require some exotic materials, about figures-of-merit (ZT) of 1.5–2, to realize performance improvements of practical utility. In contrast, topping TE generators for higher temperature range could consist of non-exotic and readily available materials with thermoelectric with ZT of unity or less, due to the larger available temperature difference. Furthermore, the energy not converted by TE generators is used for the steam turbine. However, the associated high temperatures (e.g., >800 K for the cold side) have, so far, precluded commercialization of TE topping cycles. Here we investigate thermoelectric materials and a thermal design based on the dimensions and conditions of a real boiler existing in a power plant.

Fig. 2 illustrates the system schematic of a current state-of-the-art 520 MW class power plant unit, including the cooling portion enclosed in a dashed line. This subsystem is of particular importance in minimizing water resource usage. To enable a realistic and practical evaluation, we analyze the fluid-dynamic behavior of the gas in the furnace and solve the conjugate heat transport by thermo-fluid dynamic modeling. With surface area enhancement, the TE modules are designed between the wall of the boiler and the water tubes. The TE elements are optimized locally for the simulated gas temperature profile, which is graded along the wall height of over 20 m. These basic designs and analysis enable the prediction of a realistic overall efficiency in accordance with temperature-entropy (T-s) diagram analysis for a complete superheated Rankine cycle.

Section snippets

Boiler thermo-fluid analysis

Power plants burn fossil fuel and turn water into steam, which is then used to move turbines and generate electricity. Typically, water is circulated inside tubes around the wall of the furnace (boiler). In a subcritical boiler, the water/steam mixture leaving the tube risers is separated into water and steam. The water returns to an evaporator inlet, and the steam flows into a superheater. The superheater raises the steam temperature to avoid the formation of water droplets when the

Thermoelectric topping cycle optimization

The alternate system diagram proposed in this analysis is shown in Fig. 5. The water-cooled condenser units (heat exchangers) are replaced by dry cooling air cooled condenser (ACC) units. Due to the higher condensation temperature in the ACC, the temperature of the condensed steam increases by 12–15 K [5]. According to Ref. [11], a 13.4 K temperature increase in the condensed steam temperature reduces thermodynamic efficiency by 5%. This change is shown in temperature–entropy (T-s) diagram shown

Heat exchanger enhancement at high temperatures

Fig. 7 shows a concept schematic of placing TE generators as a topping cycle between the hot gas and the bundled steam tubes on the interior surface of the boiler wall. To compensate the increasing thermal resistance when the TE generator is added, fin surface is considered to enhance the heat transfer coefficient so that total thermal resistance from the hot gas to the steam in the boiler tube is maintained constant.

Conventional TE systems consist of many integrated elements, including

Flue gas energy recovery

Flue gas discharge (FGD) at temperatures above ambient also presents an opportunity for thermoelectric generators to harvest wasted heat energy. For a plant without a FGD scrubber, discharged flue gas is typically at 180 °C for either ordinary or supercritical boilers [39]. This temperature, when combined with cold feed water from an ACC or ambient air as a heat sink, provides a reasonably large temperature difference to drive thermoelectric generators assuming all waste heat in the flue gas is

System efficiency analysis

The following analytic modeling based on the design of thermoelectrics for the maximizing the system efficiency is plugged in a temperature–entropy (T-s) model to estimate the power plant efficiency without consuming natural water resources as shown in Fig. 5. The system efficiency is determined by total power output per heat input, the total power output counts the power from thermoelectric modules in addition to the steam turbine. The heat input to the steam turbine is reduced by the

Conclusion

This paper reports on a conceptual power-neutral method to reduce water consumption in coal-fired steam turbine power plants without sacrificing operating efficiency. The analysis model is based on a current advanced superheated steam turbine. Additional power from thermoelectric generator arrays placed on top of a Rankine cycle allows the replacement of the cooling tower condensers to the ACCs without reducing the system-level power efficiency. The ACC uses natural convection for cooling with

Acknowledgements

Authors MH and TSF gratefully acknowledge support from the US Department of Energy and General Motors under the WasteHeat 2 project. Authors KY and AS gratefully acknowledge support from the Center for Energy Efficient Materials funded by the Office of Basic Energy Sciences of the US Department of Energy.

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