Elsevier

Applied Energy

Volume 247, 1 August 2019, Pages 102-115
Applied Energy

Thermodynamic analysis of a combined cooling, heating and power system based on solar thermal biomass gasification☆

https://doi.org/10.1016/j.apenergy.2019.04.039Get rights and content

Highlights

  • Propose a novel CCHP system integrated with solar thermal biomass gasification.

  • Model solar thermal biomass gasification using thermochemical equilibrium method.

  • Present energy and exergy analysis at the design and off-design work conditions.

  • Analyze the impacts of electric load ratio and solar irradiance on hybrid system.

  • Discuss the benefits of biomass gasification assisted by solar energy.

Abstract

The objectives of this paper are to propose a novel combined cooling heating and power (CCHP) system based on solar thermal biomass gasification, and to present thermodynamic analyses to effectively improve the utilization of distributed renewable energy sources. By adopting the Engineering Equation Solver software, the thermodynamic models of components were constructed, and the simulation procedures, mainly including the heat transfer of the solar cavity receiver and the biomass gasification, were presented. Based on a 100 kW electricity capacity, the design parameters of the CCHP system were obtained to simulate and discuss the thermodynamic performances and the energy and exergy flow diagrams in the cooling and heating operation modes. The influences of the key variable parameters, including the electric load ratio and the solar direct normal irradiance in the off-design work conditions on the thermodynamic performances, were analyzed and discussed. The simulation results indicate that the CCHP system achieves average energy and exergy efficiencies of 56% and 28%, respectively, and the energy ratio of solar to biomass is approximately 0.19 in the full load operating mode. Compared to the conventional biomass gasification CCHP system without solar energy, the increasing ratio of heating value of product gas based on the solar thermal biomass gasification reaches 55.09%, and the biomass saving ratios are approximately 9.22% and 2.02% in the cooling and heating modes respectively, which indicate that the proposed hybrid system is an effective and feasible method to improve the utilization efficiency of biomass energy.

Introduction

The integration of renewable energy resources into combined cooling, heating and power (CCHP) systems is an alternative for the efficient use of distributed energy resources to reduce fossil energy consumption and carbon dioxide emissions [1]. Biomass, as a stable fuel, can be converted to gas fuel through gasification in order to replace natural gas without large modifications of prime movers or systems. In contrast with other biomass technologies, gasification technologies are able to process lower grade fuels and to improve the energy system sustainability and flexibility [2]. The CCHP systems based on biomass gasification from [3] have an equivalent energy efficiency higher than 27%, which is the minimum required by law to be included in the Spanish special production regimen. The CCHP system integrated with biomass-air gasification [4] achieves the energy efficiency of 50% in summer cooling mode and annual energy efficiency of 28%. The integrated biomass gasification CCHP system was considered to be a “high efficiency system” [3].

However, the exergy analysis of biomass-air gasification CCHP system demonstrated that biomass gasifier generally accounts for the largest exergy destruction and loss, and it was approximately 60% of total exergy destruction and loss [5]. To furthermore improve the thermodynamic performances and address some issues, a variety of methods were proposed and developed, such as biomass-steam gasification, co-firing, and assistance of solar energy. The biomass-steam gasification improves gasification efficiency and then the energy efficiency of CCHP system was improved by 31.3% in comparison to CCHP system based on biomass-air gasification in the same conditions [6]. The co-firing of natural gas and product gas through biomass gasification with their ratio of 1.0 made energy and exergy efficiencies increase by 9.5% and 14.7% and saved more primary energy [7]. The biomass and coal co-fueled gasification from the study of [8] indicated that negatively cradle-to-grave greenhouse gas emissions could be expected when biomass mass share was higher than 15%. Also, the introduction of solar energy into biomass gasification CCHP system can reduce more greenhouse gas emissions although the biomass subsystem may have greater contributions to the total system energy efficiency and exergy efficiency than those of the solar subsystem in the hybrid CCHP system [9]. Furthermore, the complement of biomass and solar energy in the hybrid CCHP system addresses the instability and discontinuity of solar energy. Consequently, the hybrid CCHP system has been one of the most promising alternatives to replace natural gas in the conventional CCHP system.

According to the interactions of the biomass and solar energy flows, the complementary methods for hybrid system configurations can be classified into two categories: parallel systems and intercross systems. The parallel system allows the solar energy and biomass energy to work separately and not to interact directly in a reaction or equipment. For example, biomass fuel and solar energy are used to drive steam turbines and preheat feed water, respectively, in a biomass-solar hybrid power plant [10], and the solar energy’s assistance achieved higher efficiency under the same heat flows and temperature ranges. The integration of solar vacuum collector into a biomass CCHP system is only used to collect heat to supplement the heat shortage of the absorption cooling and heating. A combined cycle which syngas from gasification is used in a gas turbine and solar energy is used to assist the steam Rankine cycle was investigated to be an efficient way to employ biomass and solar energy together for electricity production [11]. Also, the solar energy assists the reaction of biomass gasification, but the complementary method is not a direct interact. For example, the steam generated by solar heat collectors is sent to gasifier to assist the biomass gasification [12], [13]. Due to their separation of biomass gasification and solar energy, the configuration method of hybrid CCHP system is usually adopted according to the temperature of the integrated point of solar collectors.

The intercross system generally utilizes solar thermal energy to assist the reaction of biomass pyrolysis or gasification to produce solar fuel, which is usually called solar thermochemical application with solar higher temperatures. The solar thermal gasification has been researched from various aspects. The experiments on temperature and heating rates from [14] in biomass pyrolyze assisted by solar heat collected by a vertical axis solar furnace showed that the gas yield increased with the increase of the temperature. The performance assessments from [15] showed that solar steam-gasification of biomass makes use of concentrated solar energy to convert solid biomass feedstock into high-quality synthesis gas (syngas) – mainly H2 and CO. Kalinci et al. [15], [16] constructed a special Cassegrain optical configuration for beam-down incident solar radiation to heat the packed bed top surface in order to produce hydrogen using a biomass gasification system. The experimental results at temperatures ranging from 1273 K to 1673 K in the study of [17] demonstrated the feasibility of syngas production and the reliability of continuous biomass gasification using solar energy. The analysis of a concentrated solar supercritical water gasification of biomass [18] showed that the system-level carbon, energy and exergy efficiencies were 88%, 71% and 45%, respectively, at the optimal design point. These studies on solar thermal biomass gasification indicated that the calorific value of biomass is upgraded in an amount equal to enthalpy change of the reaction by solar input and solar energy is converted to chemical energy. The intercross of biomass and solar energy produces product gas to store so that the integration point of solar thermal biomass gasification into system configurations is usually located before the prime mover of CCHP system and it also realizes the continuous and stable operation of the hybrid CCHP system. Due to these prominent advantages of solar thermal biomass gasification, we constructed the new hybrid CCHP system on the basic researches of CCHP systems integrated with biomass-air [4] and biomass-steam [6] gasification of our research team.

The studies on hybrid CCHP systems integrated with solar thermal biomass gasification just started with the development of gasification technologies in recent years, and several different types of hybrid polygeneration or CCHP systems have been proposed and studied. A polygeneration system of generating methanol and power with the solar thermal biomass gasification was proposed and developed, which the solar incident radiation fell on the heliostats around the central solar tower and was reflected to the solar–biomass gasifier, achieved energy efficiency of 56.09% and the exergy efficiency of 54.86%, respectively [19]. A solar-biomass power generation system with two-stage pyrolysis and gasification using a line-focus solar collector and a point-focus collector [20] was proposed and assessed from its thermodynamic performances of the solar thermochemical conversion process, and it was concluded that the energy level of the solar thermal energy was improved from 0.68 to 0.90. A CCHP system driven by the solar/autothermal hybrid gasifier with an indirectly irradiative two-cavity reactor [21] was investigated and it achieved 11.5% primary energy ratio. The simulations of a synthetic CCHP based on a solar assisted biogas steam reforming process [22] using the Gibbs reactor in the ASPEN PLUS model indicated that the novel CCHP system improved annual performance by 5.4% in comparison to the reference systems that combined a biogas-fired CCHP and a solar Dish/Stirling power system. Compared to these hybrid CCHP systems integrated with solar thermal biomass gasification from [20], [21], [22], the proposed system in this paper has the following obvious differences: (1) the solar dish collector collects solar heat energy for gasifier, (2) the solar gasifier is integrated with cavity receiver for solar energy and a pipe for biomass gasification, and (3) the waste heat from product gas is used to produce steam by heat exchanger to supply biomass gasification. The performances resulted from different configurations are necessary to study and compare.

The hybrid CCHP systems were generally studied and evaluated from energy, exergy, economic and environment aspects using various performance indicators [23]. The goal of new technologies is to improve energy sustainability and decrease economic cost in applications. Due to the higher cost of renewable energy technologies, the economic analysis of hybrid system was focused such as economic feasibility and unit cost of products. However, the thermodynamic analysis using energy efficiency [24] or exergy efficiency [25] or optimization for improving thermodynamic performances is the base of feasibility of the hybrid system. To discuss the energy benefit achieved by CCHP system, primary energy saving is often employed to compare to the reference system [26]. But the performance of the reference system has direct effect on its value. For solar-assisted hybrid system, solar-to-fuel efficiency [27] and net solar-electric efficiency [28] are employed to evaluate the contribution of solar energy. The literature review revealed that most of the studies on hybrid CCHP systems are based on modelling and simulation data, and not on experimental and/or pilot installations data [2], which results from the high complexity and costs related to the hybrid CCHP experimental systems. Although the analytical models have relatively high errors compared to the experimental studies, they are useful for estimating the trend and behavior of system, as well as determining the impact of main operating factors on performance of the system. The analytical modelling and simulations in hybrid CCHP systems are acceptable.

In this study, a new CCHP based on solar thermal biomass gasification is investigated. The contributions of this work mainly are summarized as follows: (1) A novel hybrid CCHP system based on solar thermal biomass gasification is proposed for effective utilization of solar energy and biomass. This developed system reduces fossil fuel consumption and mitigates CO2 emission. (2) The thermodynamic models, especially gasifier combined biomass gasification and solar heat transfer, are constructed, and the thermodynamic performances in the different operation modes are obtained. An effective integrated utilization of the renewable energy can be achieved. (3) The impacts of key parameters on system thermodynamic performances in variable conditions are investigated and justified. Section 2 proposes the novel CCHP system based on the solar thermal biomass gasification, Section 3 constructs the thermodynamic model of key components, especially solar gasifier, Section 4 presents the energy and exergy analyses in the design and off-design work conditions and Section 5 presents some conclusions.

Section snippets

System design

The energy flowcharts of the novel CCHP system based on solar thermal biomass gasification are shown in Fig. 1. The solar dish collector reflects solar radiation (state 3) to drive the biomass steam gasification (states 1 and 2) in the solar gasifier to produce the product gas (state 4). The high temperature product gas that exited from the gasifier is used to heat tap water (state 18) in order to provide steam (state 2) for gasification, and then the product gas passes through the heat

Solar thermal biomass gasification

Fig. 2 shows the modeling procedure of solar thermal biomass gasification. There are three branches including the biomass thermochemical equilibrium, the energy balance between the biomass gasification and solar energy, and the heat transfer calculation of the gasifier. The gasification of one mole of biomass and a moles of water steam can be generally expressed as follows:CHxOyNzSw+aH2OβrcharChar+αrtarTar+rCOCO+rH2H2+rCO2CO2+rCH4CH4+rH2OH2O+rN2N2+rSO2SO2where ri is the species moles in the

Validation of models

These thermodynamic models of solar thermal biomass gasification, ICE, AC/H, were simulated in the Engineering Equation Solver (EES) software [34], and they were validated by the following methods:

  • (1)

    The ICE model, the models of the electricity generation efficiency in rating conditions and the outlet temperature of exhaust gas in the design conditions used the data from the Ref. [32]. And appropriate modification was made to them.

  • (2)

    The simulation results of AC/H model in the literature [9] were

Conclusions

In comparison to the conventional biomass gasification CCHP system, the assistance of solar thermal energy is an effective and feasible method to improve the utilization efficiency of biomass energy. The results of the case study with 100 kW electricity generation indicate that the hybrid CCHP system based on solar thermal biomass gasification achieves average energy and exergy efficiencies of 56% and 28%, respectively, and the solar to biomass energy ratio is approximately 0.19 at the full

Acknowledgements

This research has been supported by National Natural Science Foundation of China (Grant No. 51876064) and the authors acknowledge the good suggestions by the anonymous reviewers that improved the paper.

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