A combined cycle utilizing LNG and low-temperature solar energy

Highlights

• A combined cycle utilizing low-temperature solar energy and LNG together is proposed.

• Five objection functions are used to decide the best working fluids.

• Cycle with a regenerator has good performance.

Abstract

This paper has proposed a combined cycle, in which low-temperature solar energy and cold energy of liquefied natural gas (LNG) can be effectively utilized together. Comparative analysis based on a same net work output between the proposed combined cycle and separated solar ORC and LNG vapor system has been done. The results show that, for the combined cycle, a decrease of nearly 82.2% on the area of solar collector is obtained and the area of heat exchanger decreases by 31.7%. Moreover, exergy efficiency is higher than both two separated systems. This work has also dealt with the thermodynamic analyses for the proposed cycle. The results show that R143a followed by propane and propene emerges as most suitable fluid. Moreover, with a regenerator added in the cycle, performance improvement is obtained for the reduction on area of solar collector and increase on system efficiency and exergy efficiency.

Introduction

An increasing demand for energy in the emerging economics today has stimulated a growing number of researches on renewable energy like solar energy, wind energy, biomass and geothermal heat. Solar energy is classified as a low grade heat source using non-concentrated solar collectors to collect radiation.

Organic Rankine cycle (ORC) is generally used to converse low-temperature solar thermal into work or electricity due to its favorable characteristics to exploit low-temperature heat sources. Solar ORCs have been studied both theoretically and experimentally [1], [2], [3] as early as 1970s. In recent years, most studies are focused on fluid selection and optimization for different cycles. Delgado-Torres et al. [4], [5], [6] have done the research of solar ORC driven by medium-temperature and low-temperature solar collectors. Subsequently, they analyze a medium-temperature solar power cycle to a sea water reverse osmosis desalination unit to maximize the fresh water production per unit of aperture area of the solar field [7], [8]. Tchanche et al. [9] and Kosmadakis et al. [10] deal with the selection of most suitable fluids for a low-temperature solar organic Rankine cycle. Wang and Zhao [11], [12] analyze the use of pure fluid of R245fa and binary zeotropic mixtures of R152a and R245fa as working fluids of the solar ORC in a direct vapor generation configuration. However, it can be found easily from these works that the overall thermal efficiency is very low for solar power cycle.

Some experimental data from operational solar ORC systems also indicate the problem. Manolakos et al. [13], Wang et al. [11] and Kane et al. [14] study solar ORCs of different collectors with overall thermal efficiency of only 7.74%, 4% and 4.2% separately. One commercial solar power plant which is reported in the technical literature shows an overall efficiency of 12.1% for a collector efficiency of 59% [15].

As mentioned above, the key problem for solar power cycle is how to improve overall thermal efficiency. Cooling water or air is usually used as heat sink for conventional ORCs, which makes it hard to improve cycle efficiency. Thus, some researchers use the liquefied natural gas (LNG) coldness to improve the performance of conventional thermal power cycle. For example, Zhang et al. [16], Liu et al. [17], [18], Dispenza et al. [19], [20], Shi et al. [21], Song et al. [22] introduce liquefied natural gas (LNG) as heat sink in ORCs to achieve larger temperature difference and exploit the cold energy of LNG.

LNG which is at atmospheric pressure and at a temperature of −162 °C has to be re-gasified and fed to a distribution system. Sea water is used typically as the heat source to vapor LNG, which not only consumes power to drive the sea water pump but also damages the marine ecosystem. In china, the utilization of LNG and corresponding research on the cold exergy of LNG are few before 2000, whereas, as the import of LNG increase with years (from 483 tons in 2005 to 10 million tons in 2011), the utilization of cold exergy of LNG attracts more and more attention.

Obviously, looking heat sources instead of sea water to vapor LNG is needed. Analyses of using waste heat as heat source have been done in some papers [23], [24]. However, whether there is waste heat appropriable around the LNG receiving terminals is a problem. Seeking a heat source of practical for use is still a puzzle.

In china, the LNG receiving terminal is usually built in coastal cities such as Guangdong province, Fujian province and Shandong province, where the solar energy is abundant all the year around. Therefore, utilizing solar energy and LNG together in such places can not only solve the problem of low thermal efficiency for conventional solar ORC but also supply a heat source of practical for use for LNG vapor system.

In this paper, a novel combined cycle with LNG as heat sink is proposed for a better conversion of solar energy. Cycle performance comparative between the novel cycle and separated solar ORC and LNG vapor system is analyzed. Moreover, it is widely known that the working fluid candidates should be integrated into any novel system due to the strong interdependence between the optimal working fluid. Thus, this work deals with the selection of suitable fluids for this combined cycle using five objective functions. At last, cycle performance with or without regenerator is also analyzed.