Cooling Water Network Research Articles

Numerical analysis of a combined heat pump ice energy storage system without solar benefit – Analytical validation and comparison with long term experimental data over one year

Ice energy storage systems, as interface between the time-shifted occurring heat and cold demand of nonresidential buildings, are a promising technology to use waste heat arising inside buildings. Several studies examine the suitability for residential buildings in combination with solar thermal power. However, investigations of ice energy storage systems for nonresidential buildings to use waste heat instead of solar benefit are missing. In order to design, evaluate and optimize such systems, analysis of its behavior by means of variational calculations are essential, which requires an adaptable model that is thoroughly validated. Therefore, for the first time, a detailed numerical model of such a system is developed, validated and compared to long term measured data of a 500 m3 setup. The storage at the University of Bayreuth acts as heat source for a heat pump and is regenerated by the cooling water network delivering the waste heat, which can also be used as a direct source for the heat pump. The Stefan problem is utilized to conduct the validation and, moreover, a comparison with 13 months of measured data shows good agreement. In the future, the model will be used to identify favorable procedures for implementing, optimizing and controlling ice energy storage systems in nonresidential buildings with waste heat recovery capability.

Integrated Optimization Framework for the Design and Scheduling of Batch Cooling Water Networks

Cooling water system (CWS) research traditionally focuses on the interactions between the cooling water network (CWN) and cooling tower performance, with a continuous background process. Very limited research has considered the integration of batch production and utility systems. This work presents a mathematical framework for the synthesis and optimization of a CWS consisting of multiple cooling towers operating with a batch background process. The CWN exhibits a series configuration, which is characterized by cooling water reuse. A batch CWS superstructure is developed which represents all different pairings between the cooling towers and the batch background network. It is worth mentioning that in previous research, the utilization of cooling water reuse opportunities was constrained by temperature only; however, due to the nature of batch processing, the utilization of cooling water reuse opportunities is subject to both temperature and time constraints. The benefits of this research for retrofit design is the increase in cooling tower availability. In the case of grassroots design, the capital expenditure for cooling tower capacity is reduced. The mathematical model is developed as a mixed-integer nonlinear programming (MINLP) problem within the GAMS platform. The MINLP is solved by making use of the BARON solver ensuring global optimality. Two illustrative examples are presented. Results indicate a 0.45% increase in the annual profit, a reduction of 42.1% in the total recirculating cooling water requirement, and a 25.3% increase in the average overall cooling tower effectiveness. Capital costs and plant area savings were introduced through a reduction in the number of cooling towers required from 3 to 2. The makeup water consumed was also reduced by 6.3%; this reduced the operational costs and plant water footprint. One of the drawbacks of the model presented is the computational time requirement. Future work will address this concern by implementing decomposition and investigating the use of alternative batch background scheduling formulations.

Optimisation, retrofitting, and variable condition analysis of cooling water networks with improved intermediate main configurations

The cooling water network is an indispensable process in industrial waste heat treatment. Previous studies primarily focussed on optimisation to reduce total economic costs with less adjustable to variable operating conditions. This study proposes two types of improved intermediate water main configurations to optimise and retrofit cooling water networks. The optimisation target is to reduce the annual total cost, and the retrofitting target is to create the optimal adjustment by not changing the original configuration. An energy-saving strategy that combines main-auxiliary pumps and water turbines is proposed to reduce the extra cost of the static pressure head. The optimal adjustment owing to the variable number of heat exchangers and a static head is analysed. The differential evolution algorithm was applied to solve the optimisation and retrofitting problems. Five configurations were compared through optimisation results and retrofit analysis in a case study. The two configurations proposed in this study have a lower total annual cost and can be retrofitted flexibly with variable working conditions. This study proposes a novel approach for an economic optimisation and retrofitting bottleneck problems.

Hybrid nonlinear model predictive control of a cooling water network

A Hybrid Nonlinear Model Predictive Control (HNMPC) strategy is developed for temperature control and power consumption minimisation of a cooling water network. The HNMPC uses a gradient descent optimisation algorithm for the continuous manipulated variables, and an enumerated tree traversal algorithm to control and optimise the Boolean manipulated variables. The HNMPC is subjected to disturbances similar to those experienced on a real plant, and its performance compared to a continuous Nonlinear Model Predictive Control (NMPC) and two base case scenarios. Power consumption is minimised, and process temperature disturbances are successfully rejected. Monetary benefits of the HNMPC control strategy are estimated.

Simultaneous Water and Energy Saving in Cooling Water Networks Using Pinch Approach

Abstract Recirculating cooling water system is the most common practice used for rejection of waste heat to environment. The fresh cooling water is distributed directly without reuse via a heat exchanger networks in parallel configuration to provide cooling duties for hot process stream. Under this configuration, thewater returns to cooling tower with high heat capacity flow rate and low temperature. These conditions will lead to poor performance of cooling tower.In this paper, process integration approach based on Pinch analysis technique used to address energy efficiency issues in cooling water networks for retrofit design problem. The Demand-Composite Analysisis an analytical approach requiring no graphical procedures developed in this work to target simultaneously, prior to the detailed network design, the minimum heat capacity flow rate of fresh cooling water and the maximumtemperature of waste cooling water to maximize the efficiency of cooling tower.The key basis for this methodology is the conversion of the cooling duties to its equivalent demand and source pairs.Agraphical representation called Demand-Composite Curveis then generated to understand the targeting philosophy. To synthesize a network that achieves the target values, the construction ofTemperature Diagram is also proposed as a design methodology for maximum reuse of cooling water.A case study is treated to illustrate the practicability of this approach.

Optimization of circulating cooling water networks considering the constraint of return water temperature

Circulating cooling water system is widely used in industry. Traditionally, circulating cooling water system is in a parallel structure. Without reuse of cooling water, the return temperature of water is low and total flow rate of water is high. This leads to the low efficiency of cooling tower and high energy consumption of system. When the coolers are in series arrangement and water is reused, the return temperature of cooling water will increase. High return temperature of cooling water can lead to the severe fouling of coolers. The level of fouling depends on outlet temperature and velocity of water. In this paper, we proposed model where cooling water is reused and the fouling of cooler is avoided. The numerical relationship between stream velocity and return temperature is introduced. The objective is to formulate the framework with minimum flow rate and no fouling with coolers. A case study is used to show the effectiveness of the proposed model.

Dynamic modelling of induced draft cooling towers with parallel heat exchangers, pumps and cooling water network

In the process industries, cooling capacity is an important enabler for the facility to manufacture on specification product. The cooling water network is an important part of the over-all cooling system of the facility. In this paper a cooling water circuit consisting of 3 cooling towers in parallel, 2 cooling water pumps in parallel, and 11 heat exchangers in parallel, is modelled. The model developed is based on first principles and captures the dynamic, non-linear nature of the plant. The modelled plant is further complicated by continuous, as well as Boolean process variables, giving the model a hybrid nature. Energy consumption is included in the model as it is a very important parameter for plant operation. The model is fitted to real industry data by using a particle swarm optimisation approach. The model is suitable to be used for optimisation and control purposes.

Simultaneous design of pump network and cooling tower allocations for cooling water system synthesis

To avoid wasting resources and energy, a simultaneous design approach is proposed for process synthesis of cooling water system in this paper. For a cooling water system involving multiple supplies and cooling water using operations, an integrated optimization is presented in which the pump network, cooling water network and cooling tower are designed as a whole system. Mixed-integer nonlinear programming based on a superstructure description is formulated by considering the configuration of the main-auxiliary pump, the location of the cooling towers, and the supply mode of cooling water simultaneously. Four operational cases are presented and analyzed in detail for the integrated cooling water system design. In all cases, global optimality is achieved with zero integrality gap, thus indicating that the optimal location and load of each cooling tower along with the optimal configurations of the pump network and the cooling water network are obtained. Relaxation techniques for addressing the nonlinear terms in the model are also presented and good performance in computation speed can be achieved.

Synthesis of energy efficient chilled and cooling water network by integrating waste heat recovery refrigeration system

Abstract Vapor compression refrigeration system (VCRS) is the conventional technology that uses electricity to generate chilled water for process cooling and air conditioning. On the other hand, there are various alternative green technologies that use waste heat to drive refrigeration system. In current industrial practices, abundant amounts of waste heat in the form of steam, hot water and flue gas remain untapped and are wasted. Absorption refrigeration system (ARS) is the alternative green technology that could recover those waste heats to produce cooling utility. In previous works, the integration of chilled and cooling water network within an Eco-Industrial Park (EIP) has been proven to be more cost effective than individual plant. However, the network is configured with VCRS which is an energy intensive technology. In this paper, ARS is integrated with VCRS to synthesize an energy efficient chilled and cooling water network using superstructure optimization approach. To further enhance energy efficiency, secondary waste heat recovery is proposed. Results shown the proposed ARS-VCRS integrated network has reduced the CO2 emission and the overall costs by 53% and 21% compared to VCRS alone. The minimum cooling duty and waste heat in the EIP for ARS-VCRS installation are determined through sensitivity analysis.

Assessing the Operation of the Cooling Water System of a Hydro-Power Plant Using EPANET

This paper focuses on the Cooling Water System (CWS) of Vidraru Hydro-Power Plant (HPP) − a 220 MW underground HPP, on the Arges River in Romania. It is equipped with 4 high head vertical Francis turbines of 55 MW each, 4 hydropower generators of 61 MVA each, and 7 step-up transformers of 40 MVA each. The goal is to model the above CWS and to analyse the system response under normal and critical operating scenarios. The numerical model built in EPANET is complex: the cooling water network is normally fed by pumping from the underground tailrace (using up to 3 pumps of 110kW each), but there is a backup supplying circuit fed directly from the penstock bifurcation pipe (using pressure reducing valves). Within the power plant, the following equipments need cooling water: air-water heat exchangers (related to generators and HVAC system); oil-water heat exchangers (related to bearings, speed governors and transformers); coolers of the turbines seals. Since EPANET lacks such physical components, they are artificially replaced by throttle control valves set with equivalent loss coefficients, established upon pressure drop values available for each equipment. The computed flow rate and pressure values match the existing recordings for normal operation. The numerical model also provides data on the system behaviour under critical operation conditions, e.g. when the HPP is operating at full capacity with CWS fed only by 2 pumps, out of 3 available pumps.

Multi-objective optimization for resource network synthesis in eco-industrial parks using an integrated analytic hierarchy process

The establishment of eco-industrial parks (EIPs) is an effective approach to developing industrial symbiosis in which plants of different entities could reduce waste production through co-sharing common utilities and/or resources, thus minimizing environmental impacts. Common resources such as heat and water are often the precursor of forming the symbiotic resource exchange network in an EIP. However, synthesizing the resource network in an EIP is a complex decision-making process involving the optimization of various design criteria by a group of decision-makers with different interests. Hence, this paper presents a multi-objective optimization approach that uses numerical representation to rank the preferences of each participating plant on the predefined criteria for building an EIP. The analytic hierarchy process (AHP) is a well-established decision-making technique for the analysis of complex decision problems. With its mathematical simplicity and flexibility, AHP can be integrated with different optimization techniques, such as mathematical programming. In this work, we develop an integrated AHP (IAHP) approach to the formation of an optimum resource network in an EIP. The optimization model is formulated to obtain a balanced solution that achieves the optimum design criteria defined by a given set of constraints. The design criteria considered in this work include economic performance, environmental impact, connectivity and network reliability. A case study on the synthesis of a chilled and cooling water network is used to illustrate the proposed methodology.

Numerical Simulation of the Cooling Water System of a 115 MW Hydro-Power Plant

The present case study points on the cooling water system of the Hydro-Power Plant (HPP) Bradisor, on the Lotru River in Romania. This 115 MW underground power plant is equipped with two Francis turbines. We built in EPANET a numerical model to simulate the operation of the cooling water system of this HPP, for different working scenarios. The whole hydraulic system is a complex one: it is composed of a main cooling water network fed by pumps, a water-based fire fighting network for the hydropower generators and a backup cooling water network fed from the penstock pipes. The numerical model of the whole cooling water system consists of 87 pipes, 133 junctions, 6 variable speed driven centrifugal pumps, 59 valves and 3 reservoirs. Some throttle control valves (TCV) are artificially inserted in our numerical model and are set with appropriate loss coefficients values to be equivalent to the equipments that introduce minor losses in the system and don’t exist as physical components in EPANET. Those equipments are: air-water heat exchangers (placed around the hydropower generators), oil-water heat exchangers (of heavy-duty thrust bearings, of turbines’ and generators’ guide bearings, of the 130 MVA step-up transformer), coolers of turbines’ seals and self-cleaning filters (one for each hydropower unit and another one for the transformer). The loss coefficient for each equivalent TCV was set based on pressure loss values measured in situ on each equipment. The numerical results (pressure and flow rate values) match the existing recordings. The simulations prove that by using a proper calibration of the numerical model, EPANET capabilities can be extended for more complex systems than the water distribution systems.

Incorporating Timesharing Scheme in Ecoindustrial Multiperiod Chilled and Cooling Water Network Design

Chilled/cooling water is a common utility widely used in office or commercial buildings for air conditioning. It is also used in various industries such as semiconductor fabrication and the food industries for process cooling. Previous studies focused on the synthesis of chilled/cooling water networks within individual plants. The establishment of an ecoindustrial park (EIP) that originates from the concept of industrial symbiosis allows individual plants to cooperate with each other so as to achieve greater water and energy recovery through interplant chilled/cooling water reuse/recycle. However, periodical circumstances such as variation of market demands and plant shutdown schedules, which are common in different business operations, have not been well considered in designing an EIP. Thus, there is a need to study explicitly the effect of such periodical operations due to the high level of connectivity within an EIP. This paper presents a design methodology to develop a robust EIP by integrating a set ...

Superstructural approach to the synthesis of free-cooling system through an integrated chilled and cooling water network

Chillers are major energy consumers in industrial facilities. They are indispensable in such industries as semiconductor fabrication, food processing, and plastics manufacturing, among others. Previous studies aimed at improving the energy efficiency of chilled water systems have focused on optimizing the performance of individual chillers. However, an alternative method to recover energy is to perform system-wide water source/sink integration using a superstructural approach. In our previous study, several schemes for chilled and cooling water systems (CCWS) with hub topology were proposed for energy savings. The main contribution of this study is in the development of a methodology to achieve energy savings by introducing free cooling in an integrated superstructure for CCWS. Two examples are used to demonstrate three different scenarios of CCWS with free cooling. It is shown that the integration of free cooling into chilled water system improve the cost and energy saving significantly, and can avoid the need to invest in a new chiller and/or cooling tower to enhance the energy efficiency of CCWS.

Fuzzy analytic hierarchy process and targeting for inter-plant chilled and cooling water network synthesis

In recent years, the concept of industrial symbiosis (IS) has led to improvements in resource efficiency that may not be possible with individual industrial plants acting independently. One specific aspect is to achieve economies of scale by having multiple companies located in close proximity in order to share common utilities, such as chilled and cooling water. Together, these industrial plants may form an inter-plant chilled and cooling water network (IPCCWN) to achieve greater overall cost savings. Some issues faced by an IPCCWN include network reliability problems due to the consistency of sources' availability, and cost savings allocations for IPCCWN synthesis due to the subjectivity of human preference on decision making. Thus, there exists a need for a decision-making tool to determine a feasible solution that will satisfy all industrial plants in the IPCCWN. In this work, a multi-objective linear programming model is developed to synthesize IPCCWN that achieves maximum cost savings. Next, a Pareto optimal solution is selected using fuzzy analytic hierarchy process (FAHP) approach. This solution gives the best balance of performance for a set of pre-defined qualitative and quantitative criteria, which is able to account for subjectivity that cannot be addressed from a purely mathematical programming standpoint.

English

An important area in process integration is the development of methodologies to minimize water and energy use in industry. More than 20 % of the energy consumption in industry is associated with cooling and heating water. This paper presents analysis and optimization of a re-circulating cooling water system, with the aim to satisfy any supply conditions for the cooling tower. The part of the atmospheric crude oil distillation unit was chosen for analysis and synthesis of cooling water systems by the Kim and Smith design (KSD) method. The load of the cooling tower and the cost related to the cooling water system could be reduced by modifying the configuration of the heat exchanger network. In this paper, the KSD methodology for a developed heat exchanger network is expanded with the principle based on the heuristic algorithmic water sources diagram procedure (WSD) to synthesize the mass exchange network. These procedures are advantageous compared with other methodologies since hand calculation is used, a very useful feature for process engineers. The cooling water network was synthesized, leading to a 40 % reduction in cooling tower load, and consequently, lower operating costs and water consumption.

Pump network optimization for a cooling water system

Centrifugal pumps are widely used in cooling water systems to transport cooling water to its users. They are installed in the header line of the feed pipe, constituting a main pump network. The pressure head of the main pumps must be large enough to satisfy the pressure heads of all coolers. The pressure drop of parallel branch pipes must be balanced by reducing the opening of valves for some coolers, incurring an energy penalty on some pumps. To attain energy savings, this paper proposes an auxiliary pump network whereby auxiliary pumps are installed in parallel branch pipes. A superstructure-based mathematical model is developed to optimize the total cost of the main and auxiliary pump networks. The optimal number of auxiliary pumps and their installation locations are determined by solving the model with a simulated annealing algorithm. The effectiveness of the model is tested by a case study based on the cooling water network of a refinery.

Two-Step Methodology for Retrofit Design of Cooling Water Networks

Conventional cooling water networks usually operate in a parallel configuration. Converting a parallel design to a series arrangement can yield a significant reduction in fresh cooling water consumption because of water reuse. This paper presents a two-step methodology for retrofit design of cooling water networks. The proposed methodology can convert a parallel configuration to a series arrangement without investment in a new heat-transfer area. The series arrangement is assumed to contain two heat exchangers. The first heat exchanger in the series arrangement is designated as the supplier because it supplies reuse water to the second heat exchanger, which is called the receiver. To retrofit a cooling water network with a parallel arrangement of several heat exchangers, the first step of the methodology uses the concept of flow rate difference curve to classify the heat exchangers into the two categories of supplier and receiver. In the second step, receiver sensitivity graphs provide guidelines for systematic matching of the suppliers and receivers. A case study is presented to illustrate the practical utility of the two-step methodology.

Holistic Approach for Retrofit Design of Cooling Water Networks

Optimization of closed-loop cooling water systems (CCWS) is essential to reduce operating and capital costs. These systems are generally designed with heat exchangers in parallel arrangement. Although this arrangement is easy to operate, it results in inefficient use of cooling tower capacity and incurs more capital and operating costs. Existing CCWS can be retrofitted by changing heat exchangers from parallel to series or series/parallel arrangement. Previous research on cooling water retrofit design is mainly focused on methods to reduce cooling water flow rate without considering cooling tower operation, pressure drops, and capital costs. With pressure drop and capital cost considerations, minimizing cooling water flow rate does not necessarily give the optimum savings. In the present paper, a holistic approach for retrofit design of CCWS involving multiple cooling towers and heat exchangers is presented by considering exchanger network pressure drops, cooling tower operation, and piping costs besides ...

Targeting and design of CWNs (cooling water networks)

CWNs (cooling water networks) are targeted and designed in this work by an analytical approach requiring no graphical procedures. The key basis for the approach is the conversion of the cooling duties to equivalent inlet–outlet (demand–source) pairs. The UTA (Unified Targeting Algorithm) is first used to establish the minimum CW (cooling water) flowrate and identify the pinch temperature. The Enhanced NNA (Nearest-Neighbors Algorithm) is then utilized to systematically synthesize CWNs by giving priority to LR (local-recycle) matches. The flowrate of the LR match is maximized wherever possible by choosing the neighbor source as the cleanest available source rather than the nearest neighbor. This is advantageous because LR matches can be eliminated for fixed load operations to yield relatively simple network designs that not only meet the minimum CW flow target but also minimize the flowrate through the coolers resulting in reduced network cost. Two case studies are presented to illustrate the versatility of the approach in generating multiple optimum network designs for maximum reuse, temperature constraints and debottlenecking. Importantly, it is demonstrated that superior practical networks can be designed for temperature constrained CWNs with pinch migration as well as without pinch recognition.