Santanu Bandyopadhyay

Multiple utilities targeting for heat exchanger networks

A targeting methodology is proposed to determine the optimum loads for multiple utilities considering the cost tradeoffs in energy and capital for heat exchanger networks (HENs). The method is based on a newly-developed Cheapest Utility Principle (CUP), which simply states that it is optimal to increase the load of the cheapest utility and maintain the loads of the relatively expensive utilities constant while increasing the total utility consumption. In other words, the temperature driving forces at the utility pinches once optimized do not change even when the minimum approach temperature (ΔTmin) at the process pinch is varied. The CUP holds rigorously when the relationship between the exchanger area and the capital cost is linear. Even when the relationship is non-linear, it proves to be an excellent approximation that reduces the computational effort during multiple utilities targeting. By optimizing the utility pinches sequentially and recognizing that these optimized utility pinches essentially do not change with the process ΔTmin, the results can be elegantly represented through the optimum load distribution (OLD) plots introduced in this work. The total annual cost (TAC) target curves can be then established from the OLD plots for pre-design screening of various options that lead to near-minimum cost HENs but involve different combinations of utilities and load distributions. Rather than determine that single value for the global optimum corresponding to the minimum TAC, it is beneficial in practice to define an optimum ΔTmin range because the TAC curves are often reasonably flat in the neighbourhood of the minimum and consequently provide useful flexibility in terms of capital investment.

One-Cycle-Controlled Single-Stage Single-Phase Voltage-Sensorless Grid-Connected PV System

Reforms in the electricity sector along with various renewable-energy-promotion policies have increased the importance of small grid-connected photovoltaic (PV) systems utilizing single-stage single-phase inverters. Ruggedness, reliability, and cost effectiveness are the desirable characteristics of such inverters used in distributed low-power applications. Schemes based on one-cycle control (OCC) which do not require the service of a phase-locked loop for interfacing the inverter to the grid are increasingly being employed for such applications. However, the OCC-based schemes reported earlier require sensing of the grid voltage which somewhat offsets one of the inherent strengths of OCC-based systems. In an effort to overcome the aforementioned limitation, an OCC-based grid-connected single-stage PV system is proposed in this paper which does not require to sense the grid voltage. Further, it requires less number of sensors (two) as compared to that required (four) in the earlier reported scheme for the implementation of the core controller comprising of OCC and maximum-power-point-tracking blocks. The viability of the proposed scheme is confirmed by performing simulation and experimental validation.

Temperature–enthalpy curve for energy targeting of distillation columns

Abstract The temperature–enthalpy ( T – H ) diagram of a distillation column at practical near-minimum thermodynamic condition (PNMTC) or the column grand composite curve (CGCC) is a useful representation for energy targeting studies and may be generated from a converged simulation of a base-case column design. The calculation procedure for the CGCC involves determination of the net enthalpy deficit at each stage by generating envelopes from either the condenser end (top-down approach) or the reboiler end (bottom-up approach). However, the values calculated by the two approaches differ for stages with feeds because existing procedures for CGCC generation do not consider the enthalpy balances at the feed stages. In fact, the net enthalpy deficits at feed stages calculated by both approaches are erroneous even for the simplest case of binary distillation. A feed stage correction (FSC) that rigorously considers the mass and enthalpy balance equations at feed stages is proposed in this work to resolve the discrepancy. Instead of assuming that the compositions obtained from the converged simulation for a feed stage will remain unchanged at PNMTC, the pinched compositions for the feed are determined by the intersection of the equilibrium curve and the feed q -line. Rather than perform an additional flash calculation to establish the pinched feed compositions, a quadratic approximation is developed here for column targeting purposes by assuming the relative volatility obtained from the simulation to remain constant in the neighborhood of the feed stage. The proposed FSC ensures that the CGCC is identical whether the calculations are performed by the top-down approach or the bottom-up approach. The effect of the FSC on the targets for energy conservation by reflux modification, feed conditioning, and introduction of side reboilers/condensers is discussed. As the energy target for reflux modification is determined by the CGCC pinch which typically occurs at or close to the feed location, the significance of the FSC on the reflux modification target is highlighted through several case studies including a complex column featuring multiple feeds and consequently multiple pinch points. The CGCCs for these case studies are generated by a computer program based on the FSC and a single analytical equation for the calculation of the net enthalpy deficits that allows every stage to have a feed, liquid product, vapor product, and side exchanger. The studies show that the reflux modification targets may be erroneous in many cases, if the FSC is ignored.

Emission constrained power system planning: a pinch analysis based study of Indian electricity sector

In the light of rising electricity demands and a need to curb carbon dioxide emissions, this article investigates the problem of power system planning with emission targeting. A pinch analysis based approach is utilised here. The key aspect of this study is investigating the parameters that decide the priority of one type of power plant over another. For this, a quantity called prioritised cost, a trade off between cost incurred and emission from a new power plant is identified. In addition to cost and emission factor of a power plant, a third parameter, the present state of the system, also plays a significant role in deciding a power plant’s prioritised cost. The analysis done proves that new power plants can be added to the system in the order of their prioritised cost. This methodology is applied to Indian power sector as a case study. Two different problems, involving minimisation of investment and annualised cost, are considered. It is observed that renewables are slightly more favoured when the objective is to minimise overall cost and not just the capital investment. In both cases, the energy mix is still dominated by coal-based power generation. The share of renewables was seen to increase with more stringent emission targets when the objective was to minimise overall cost.

Effect of feed on optimal thermodynamic performance of a distillation column

Abstract Invariant rectifying-stripping (IRS) curves are presented for thermodynamic analysis of distillation column. The IRS curves are invariant to the column configuration (i.e., feed location and total number of stages) and therefore, they are useful to set quantitative targets such as minimum energy requirement (minimum condenser and reboiler duties), appropriate feed location, proper feed preconditioning, scope for side-condensers/reboilers, as well as thermo-economic optimization of distillation column. In this paper, the relation between the IRS curves and exergy loss in an adiabatic distillation column is established through a simple graphical representation. A general methodology is put forward to analyze the optimal thermodynamic performance of a distillation column. Effects of feed composition, thermal condition of the feed, relative volatility of components, and sharpness of separation on the thermodynamic efficiency of a distillation column are presented in this study. The methodology, described in this paper, is equally applicable to real systems, with or without tangent pinch. A general principle for feed preheating and side-exchanging is also developed.

Thermoeconomic optimization of combined cycle power plants

Recognition and acceptance of irreversibility in the design and operation of any power plant is essential due to the constraints in finite resources (such as finite time of operation, finite size of plant, finite capital investment, finite power production, etc.). Thermoeconomic optimization of a combined cycle power plant, comprised of an arbitrary number of internally irreversible Carnot-like heat engines, is studied in this paper considering finite resource constraints. The efficiency of a multistage endoreversible combined cycle power plant corresponding to maximum power production or minimum operating cost is observed to be identical to that of a single endoreversible heat engine under the same operating conditions. An increase in the number of stages reduces power production but increases total annualized cost of the plant. Inventory control of heat exchanger surface areas or their thermal conductances and the directions for heat transfer augmentation to get maximum benefits are also discussed in this work. Flexibility in selecting different working fluids at different operating pressures is identified for optimal design and operation of the combined cycle power plant.

Sizing curve for design of isolated power systems

Isolated power systems meet electricity demand by generating power close to its point of utilisation. They are an option to electrify communities located in remote areas where extending the grid could be uneconomic. Diesel generators, photovoltaic panels and energy storage using battery banks have been used for meeting the electrification needs of remote areas. The design objective for such systems is the estimation of the ratings of the generators and the storage capacity requirements for meeting specified reliability and economic constraints. A review of different methods for sizing photovoltaic-battery systems indicates that they fall into mainly two categories, analytical methods and simulation-based schemes. A generalised methodology for generating a “sizing curve” relating the generator rating and storage capacity, based on a time series simulation approach, is presented in this paper. It helps in the identification of a “design space” which enables the exploration of all the feasible system configurations meeting a given demand for a site. It further serves as a tool for system optimisation. Two specific options for isolated power generation, diesel generator-battery system and photovoltaic-battery system, are illustrated for a typical Indian site. Sizing curve and design space are plotted on normalised generator rating vs. storage capacity coordinates for these options.

Invariant rectifying-stripping curves for targeting minimum energy and feed location in distillation

Invariant rectifying-stripping (IRS) curves are proposed that are independent of the feed location and operating reflux of the distillation column for a given separation problem. IRS curves represent the enthalpy surpluses and deficits in the rectifying and stripping sections, respectively, as a function of temperature for all possible values of reflux and reboil. The IRS curves provide a new representation on the temperature-enthalpy diagram to set distillation column targets prior to detailed design for minimum energy requirement, feed location, feed preconditioning, and side-exchanger loads. The application of the proposed concepts to two binary distillation examples (one featuring a tangent pinch) and a multicomponent distillation example illustrates the usefulness of the IRS curves in properly locating the feed, determining the minimum utility requirements, and reducing the tedium of repeated simulations. The IRS curves are rigorously invariant and provide the absolute minimum utility requirements for binary systems (ideal as well as non-ideal); however, they are near-invariant and predict the near-minimum utility requirements for multicomponent systems (where the pseudo-binary concept of a light and heavy key is employed).

An Active Harmonic Filter Based on One-Cycle Control

The widespread usage of load compensators for carrying out harmonic and reactive power compensation is constrained by cost and poor efficiency due to switching losses. Hence, it is customary to employ active harmonic filters for harmonic compensation only, while traditional methods comprising of thyristor-switched capacitors are used to carry out reactive power compensation. Load compensators based on one-cycle control (OCC) have gained considerable significance as they do not require the service of a phase-locked loop to synchronize with the utility grid. However, existing OCC-based load compensators do not have the capability to differentiate between the fundamental reactive component and harmonic components of the load currents. Hence, they cannot be employed for harmonic compensation alone as they end up compensating for reactive current as well, leading to an increase in the converter rating. In order to overcome the aforementioned limitation, an OCC-based shunt harmonic filter which is capable of compensating only the harmonic components of the load current is proposed in this paper. The viability of the proposed scheme is confirmed by performing detailed simulation studies and experimental validation.

Analysis of high temperature thermal energy storage for solar power plant

Integration of storage system plays an important role for economic success of solar thermal power plant. At present two-tank, thermocline, concrete, castable ceramic and phase change material (PCM) are most common existing storage options, each of these storage system have own unique feature. A comparative analysis is done for the storage system by fixing the size of solar power plant of 50MW and storage duration of 6 hour. A cost and energy analysis is carried out to evaluate economic and energy requirement of storage system. The study evaluates the specific cost of each system. From this analysis thermocline thermal storage is come as preferred option. The performance parameter like efficiency, Levelized electric cost (LEC) are also been evaluated in this study.