UA and pinch point temperature difference modeling — Finding the best heat exchanger schemes

Process models using simplified heat exchanger (HE) models are often analyzed using methods based on derivatives or optimization procedures, where even small numerical errors can cause algorithms to fail. This article explores the use of numerical approximations for calculating pinch temperature and UA-value, including novel high-order polynomial methods using equidistant and Chebyshev grids. The results show that the mainstream methods, where LMTD and pinch temperature are calculated from grid values, are 2–5 times slower than the high-order methods if requiring accuracy better than 1%. If a 0.01% accuracy is needed high-order methods are often 10–20 times faster. Numerical errors in high-order schemes with pure fluids converge quickly to zero when increasing the grid size, and schemes with more than 30 grid points generate errors less than 1E-4%. High-order methods were less successful for fluid mixtures, where a novel hybrid high and low-order pinch temperature scheme is recommended

Comparing optimization schemes for solving case studies with multiple heat exchangers using high-order pinch point temperature difference methods

Heat exchangers (HEs) are often modeled using pinch point temperature difference (ΔTpinch) methods when optimizing systems with HEs. However, even small inaccuracies in model predictions of HEs will introduce numerical noise that can cause optimization algorithms to fail. A recent study of single HEs suggests that highorder interpolation methods can compute ΔTpinch much faster than conventional methods. However, the performance of such methods when optimizing HE systems have not previously been tested. Heat pumps with 2 and 3 HEs, with and without an ejector are optimized using different schemes. Results from these case studies show that non-linear constrained gradient-based optimization algorithms are more than 5 times faster than particle swarm (PS), and that the conventional genetic algorithm (GA) should not be used. However, the main conclusion is that the case study optimizations are solved 5–10 times faster if ΔTpinch is calculated using hybrid high and low-order interpolation methods

A comparison of the energy consumption for CO2 compression process alternatives

The efficient transportation of large volumes of CO2 generally requires pipelines that will operate above the critical pressure of CO2. Since most capture processes release CO2 at low pressure, compression of CO2 from the point of capture to pipeline will generally be required. The compression duty can be achieved using conventional multi-stage compressors or using newer shockwave type compressors. Pumping could also be used if CO2 is condensed below its critical point. This paper presents a comparison the energy consumption associated with these compression process alternatives. The focus of the review is on the clarity of the comparison and the careful optimisation of each of the scheme considered. The main finding is that the performance advantages claimed for improved CO2 compression process schemes are often optimistic because the based-line scheme compared against is not well optimized

Optimization of the Energy Consumption of a Carbon Capture and Sequestration Related Carbon Dioxide Compression Processes

It is likely that the future availability of energy from fossil fuels, such as natural gas, will be influenced by how efficiently the associated CO2 emissions can be mitigated using carbon capture and sequestration (CCS). In turn, understanding how CCS affects the efficient recovery of energy from fossil fuel reserves in different parts of the world requires data on how the performance of each part of a particular CCS scheme is affected by both technology specific parameters and location specific parameters, such as ambient temperature. This paper presents a study into how the energy consumption of an important element of all CCS schemes, the CO2 compression process, varies with compressor design, CO2 pipeline pressure, and cooling temperature. Post-combustion, pre-combustion, and oxyfuel capture scenarios are each considered. A range of optimization algorithms are used to ensure a consistent approach to optimization. The results show that energy consumption is minimized by compressor designs with multiple impellers per stage and carefully optimized stage pressure ratios. The results also form a performance map illustrating the energy consumption for CO2 compression processes that can be used in further study work and, in particular, CCS system models developed to study performance variation with ambient temperatur

A comparative study of CO2 heat pump performance for combined space and hot water heating

Heat pumps used for combined space and hot water heating are often used in modern energy-efficient buildings. Performance depends on both system design (exchanger sizes, compressor efficiency, etc.) and operating conditions (inlet water temperature, ratio between space and water heating, etc.). Designs using both CO2 and HFCs, such as R410A, are available, but the relative performance of these is not extensively studied. This article presents performance results based on a system model developed in MATLAB where exchangers are modelled using fixed temperature pinches and pressure drops. Ejectors and compressors are modelled using defined efficiencies. Operating parameters are optimized using the Genetic Algorithm for a set of sensitivity studies. The results show that CO2 can outperform R410A when the ratio of space to water heating is below 0.6 – 1.0, feedwater is below 20°C (as in norther Europe) and heat exchangers are designed with temperature pinches below 10 K

Optimization of the CO2 Liquefaction Process-Performance Study with Varying Ambient Temperature

In carbon capture utilization and storage (CCUS) projects, the transportation of CO2 by ship can be an attractive alternative to transportation using a pipeline, particularly when the distance between the source and usage or storage location is large. However, a challenge associated with this approach is that the energy consumption of the liquefaction process can be significant, which makes the selection of an energy-efficient design an important factor in the minimization of operating costs. Since the liquefaction process operates at low temperature, its energy consumption varies with ambient temperature, which influences the trade-off point between different liquefaction process designs. A consistent set of data showing the relationship between energy consumption and cooling temperature is therefore useful in the CCUS system modelling. This study addresses this issue by modelling the performance of a variety of CO2 liquefaction processes across a range of ambient temperatures applying a methodical approach for the optimization of process operating parameters. The findings comprise a set of data for the minimum energy consumption cases. The main conclusions of this study are that an open-cycle CO2 process will offer lowest energy consumption below 20 °C cooling temperature and that over the cooling temperature range 15 to 50 °C, the minimum energy consumption for all liquefaction process rises by around 40%

Optimization of a mixed refrigerant based H2 liquefaction pre-cooling process and estimate of liquefaction performance with varying ambient temperature

Hydrogen used as an energy carrier can provide an important route to the decarbonization of energy supplies, but realizing this opportunity will require both significantly increased production and transportation capacity. One route to increased transportation capacity is the shipping of liquid hydrogen, but this requires an energy-intensive liquefaction step. Recent study work has shown that the energy required in this process can be reduced through the implementation of new and improved process designs, but since all low-temperature processes are affected by the available heat-sink temperature, local ambient conditions will also have an impact. The objective of this work is to identify how the energy consumption associated with hydrogen liquefaction varies with heat-sink temperature through the optimization of design parameters for a next-generation mixed refrigerant based hydrogen liquefaction process. The results show that energy consumption increases by around 20% across the cooling temperature range 5 to 50 °C. Considering just the range 20 to 30 °C, there is a 5% increase, illustrating the significant impact ambient temperature can have on energy consumption. The implications of this work are that the modelling of different liquified hydrogen based energy supply chains should take the impact of ambient temperature into account

Optimization Study of Heat Pumps Using Refrigerant Blends – Ejector Versus Expansion Valve Systems

This article investigates tap water heating systems to highlight an ongoing debate. Some report that CO2-based transcritical heat pumps with an ejector have the best coefficient of performance (COP), while others report that blend-based refrigerant systems (without an ejector) are better. In the literature, however, these systems are only compared with conventional heat pump designs, and not against each other, making it difficult to conclude which design is the best. In addition, the outcome of combining the two modifications has not been explored extensively. This article investigates the performance of heat pumps using mixtures of CO2 and propane, with and without an ejector or a suction gas heat exchanger. It presents a novel method for modeling blend-based heat pumps with an ejector using an optimization approach and a minimum allowed temperature pinch in heat exchangers. A sensitivity study explores how the heat pump performance depends on operating conditions, ejector efficiency and the refrigerant blend. The sensitivity studies allow for the comparison of the heat pump designs. For example, the results show that it is inefficient to use CO2 and propane blends in systems with an ejector. A blend-based system with a suction gas heat exchanger was found to outperform a CO2-based system with an ejector if either the tap water is above 25°C, the ejector efficiency is below 0.17, or the temperature of the heat source is reduced with more than 10 K when flowing through the evaporator

Optimization of Pure-Component LNG Cascade Processes with Heat Integration

Liquefaction of natural gas is an energy-intensive process in which the energy efficiency depends on the number of compressors stages and the heat integration scheme. The aim of the study is to systematically evaluate process performance of pure component cascade processes, present optimized designs for all relevant numbers of compression stages and compare energy consumption between processes with differing levels of complexity. An original method for the evaluation of process performance is developed that utilizes as little human interaction as possible, making it suitable for optimization. This study shows that a pure-component cascade process using the three refrigerants R290, R1150 and R50 must have at least 11 stages to equal the energy efficiency of the best mixed refrigerant process. An optimized configuration for an 11-stage process scheme operating at 20◦C ambient temperature is described in detail