Short-term Thermal Energy Storage Research Articles

Strengths, weaknesses, opportunities and threats of demand response in district heating and cooling systems. From passive customers to valuable assets

Buildings can deliver short-term thermal energy storage by utilising the thermal capacity of the building construction and/or by activating the water tanks included in the heating/cooling installation. The flexibility potential of demand management using decentralized thermal energy storage has been quantified in many theoretical modelling studies, and it is considered an essential technology for an affordable energy transition. We have investigated the drivers and barriers to the adoption of demand management in buildings in district heating and cooling systems via a Strengths, Weaknesses, Opportunities and Threats (SWOT) analysis and presented 17 elements that shape the current and future application of this concept. The results indicate that the application of the DR concept has left the theoretical studies and moved towards real-life applications. Yet, there is a lack of feasible business models and regulatory frameworks supporting the large-scale application of the concept. Utilities and their customers do not fully understand the benefits of the DR concept; therefore they are reluctant to adopt it outside of the research projects where the test environment is fully controlled and with limited impact and timeline. Therefore, the regulatory framework must be adjusted to allow DHC operators to develop new business models and DR tariffs that will incentivise the customers to deliver flexibility to the system without compromising their comfort and everyday practices and increasing energy poverty.

Robust MPC with Support Vector Clustering-based Parametric Uncertainty Set for Building Thermal Control

Control systems are essential to support the use of building structures as short-term thermal energy storage (TES). Due to modeling and forecast imperfections, the controller must be able to deal with uncertainties. This paper proposes a robust model predictive controller (MPC) with a new uncertainty set construction technique to regulate the heat supply in a building envelope. We extend the Support Vector Clustering-based set construction technique to estimate modeling and forecast uncertainty sets. Subsequently, we integrate the sets into a Min-Max MPC framework to ensure robust feasibility by tightening the constraints. The resulting controller successfully deals with modeling and forecast uncertainties. The quality of the presented framework is compared with a nominal MPC and a robust MPC with different uncertainty set estimates. On the basis of a numerical simulation, we demonstrate that the proposed controller successfully maintains the room temperature within the comfort limits. The result also shows that our MPC is less conservative than the controller designed using a box-shaped non-falsifed parametric uncertainty set.

Do the customers remember? The fade-out effect from the demand response applied in the district heating system in Denmark

Buildings can deliver short-term thermal energy storage to energy systems. In district heating (DH) systems, it is mainly desk studies and simulations that reveal a large thermal flexibility potential. Knowledge from real-life case studies on how residents participate in demand management campaigns is crucial for the successful utilisation of buildings’ flexibility potential for minimizing bottlenecks in the daily operation of DH systems. In the field study including 72 single-family houses connected to the 3GDH network in southern Denmark, the demand response (DR) strategy “night setback” was applied for two heating periods. The houses were equipped with control and monitoring equipment, which allowed the deactivation of the heating system while monitoring the indoor temperature, so it does not drop below the defined value. The occupants controlled the DR events settings and could at any time stop utilisation of the night setback strategy (implicit participation in the DR). All 72 houses applied the night setback during both heating periods. Yet, the participation time decreased from 89% to 81%. The lowest participation rate was noted for the farm house, 60% and 9% of heating periods 1 and 2, respectively. In around 60% of the DR events, the night setback strategy was activated at 20:00.

Optimal Planning of Integrated Electricity and Heat System Considering Seasonal and Short-Term Thermal Energy Storage

Optimal Planning of Integrated Electricity and Heat System Considering Seasonal and Short-Term Thermal Energy Storage

Peak Shaving of a District Heated Office Building with Short-Term Thermal Energy Storage in Finland

Short-term thermal energy storage techniques can be effective to reduce peak power and accommodate more intermittent renewable energies in district heating systems. Centralized storage has been the most widely applied type. However, in conventional high-temperature district heating networks, substations are typically not equipped with short-term thermal energy storage. Therefore, this paper investigated its peak shaving potential. A 5 m3 thermal storage tank directly charged by the district heating supply water was integrated into a substation of a Finnish office building. The substation with the stratified storage tank and the office building were modeled and simulated by IDA ICE. Different storage tank temperature control curves were designed to charge the tank during off-peak hours and discharge to reduce the high-peak-period heating power. Moreover, the peak power was further dimensioned by reducing the mass flow of the primary district heating supply water. The results indicate that the storage tank application significantly decreases the office building daily peak power caused by the ventilation system’s morning start during the heating season. It reflected a higher peak shaving potential for colder days with 31.5% of maximum peak power decrease. Cutting the mass flow by up to 30% provides an additional peak power reduction without sacrificing thermal comfort.

Machine learning for performance prediction in smart buildings: Photovoltaic self-consumption and life cycle cost optimization

The application of Photovoltaic (PV) system in buildings is growing rapidly in response to the need for clean energy sources and building decarbonization targets. Nonetheless, enhancing PV self-consumption through technical solutions such as Energy Storage Systems (ESS) is getting higher importance to increase the profitability of PV plants, by minimizing the building-grid interaction. In this context, analyzing PV self-consumption of different energy storage configurations becomes more relevant and crucial in building energy modeling although it is heavily time-consuming and complicated, particularly within a multi-objective optimization related to the ESS design. As a solution to resolve this issue, this paper evaluates the accuracy, training, and prediction speed of 24 Machine Learning (ML) models to be used as surrogate models for analyzing PV self-consumption in smart buildings. Furthermore, the performance of short-term Thermal Energy Storage (TES) to increase PV self-consumption is assessed and presented using ML models. The results showed the Gaussian Process Regression (GPR), Neural Networks (NN) including bilayered and trilayered NN models, Support Vector Machines (SVM) including the fine gaussian and cubic SVM models, and Ensembles of Trees (EoT) as superior ML models. The results also revealed that TES systems can efficiently increase PV self-consumption in the building equipped with electric heat pumps to provide heating, cooling, and domestic hot water. Moreover, the TES size optimization regarding the Life Cycle Cost (LCC) showed that the LCC-based optimum TES size can yield 7.1% savings within 30 years of the building service life. The novelties of this research are first to provide a reference to select the most suitable ML models in predicting PV self-consumption, second to implement machine learning for analyzing the performance of short-term thermal energy storage to enhance PV self-consumption in buildings, and third to carry out an LCC-based optimization on TES size using ML-based prediction models.

Reducing energy consumption of heat pumps by introducing a thermal energy storage system

We proposed a short-term thermal energy storage system by applying Phase Change Materials (PCMs) in air ducts connected to an indoor air conditioning unit within a ceiling space. PCM tubes were arranged in a staggered layout in the air duct and cooled by chilled air from the air conditioning unit during “charging” mode, whereas the PCM tubes released cold energy to the room space without operation of the compressor during “discharging” mode. The charging mode allowed for an increase in the partial load ratio of the heat pump to prevent low-efficiency operation at <20% load, which can increase seasonal energy efficiency. We constructed an energy simulation program, where the heat exchange coefficient between the PCM tubes and air was varied with airflow rate and the temperature difference between the PCM and air, in accordance with results from CFD analysis. Switching between the charging and discharging modes was based on the outlet temperature (Tout) and 30% of the rated capacity of the indoor unit (Lr). The heat balance between the charging and discharging modes was improved by this change to the system control. A higher load factor and higher COP resulted in a 7.6% reduction in energy consumption.

A niche technique overlooked in the Danish district heating sector? Exploring socio-technical perspectives of short-term thermal energy storage for building energy flexibility

This research explores socio-technical perspectives of the demand-side management strategy of using the built environment for short term thermal energy storage. Here conceptualised as a niche innovation within the Danish socio-technical district heating landscapes, the research explores potentials and limitations of this building energy flexibility strategy from the perspective of district heating sector professionals, actors at the centre of the low-carbon energy transitions. Results of the mixed-methods abductive research enquiry suggest that this energy flexibility strategy facilitates (I) solving local network congestion challenges in smaller parts of existing networks and (II) reduces needed network capacity in new heat supply areas. Sector professionals assess this (III) energy flexibility strategy as most practicable in large-scale/commercial buildings and industries. Challenges include hardware balancing, service and maintenance, and the sometimes counterproductive incentive structures among stakeholders involved. Research evidence suggests that business models appealing to environmental values and priorities may incentivise sustainable heat-use behaviours more than economic benefit alone among some groups of end users. Building energy flexibility and demand-side management strategies may become integral to future ‘smart’ energy systems throughout the world. However, their successful implementation necessitates understanding the local socio-technical dynamics involved. Multidisciplinary research approaches as the one taken here facilitate these necessary insights.

Distinguish between the economic optimal and lowest distribution temperatures for heat-prosumer-based district heating systems with short-term thermal energy storage

Decreasing distribution temperature is the key driving force for energy-efficient and economic-competitive district heating (DH) systems, powering the transition from the 1st generation high-temperature steam-based DH systems to the 4th generation low-temperature water-based DH systems. However, for heat prosumers integrated with thermal energy storages (TESs), the decreasing distribution temperature may lower the peak load shaving capability of TESs and raise the peak-load-related heating costs, due to decreased charging temperature. This study, therefore, aimed to investigate the overall impacts of distribution temperature and to identify the economic optimal distribution temperature for heat-prosumer-based DH systems integrated with short-term TES. Firstly, an economic optimization problem was developed. Afterwards, distribution temperatures were optimized within the feasible regions of individual scenarios, including three benchmark scenarios representing the 2nd, 3rd, and 4th generation DH and an improved scenario featured high feasibility in distribution temperature. A campus DH system in Norway was used as the case study. Results revealed that the improved scenario's economic optimal distribution temperature was significantly distinct from the 4th generation DH scenario's lowest distribution temperature, both in terms of distribution range and annual average level. Finally, broad conclusions were reached by discussing the impacts of crucial factors on economic optimal distribution temperatures.

A holistic design approach for 5th generation smart local energy systems: Project GreenSCIES

This paper introduces a project called GreenSCIES; an InnovateUK funded detailed design project to develop a Smart Local Energy System (SLES) delivering a significant carbon saving for local residents, schools and businesses. The SLES is centred around a 5th Generation District Heating and Cooling (5DHC) network in the London Borough of Islington. The local energy system will deliver low carbon heating, cooling, power and e-mobility charging powered by renewable energy and waste heat, sourced from the local area. The proposed SLES will manage and balance the supply, storage (both thermal and electrical) and use of local energy across mobility, power and heat vectors. It will do so using smart control technologies, bringing significant energy efficiency and security as well as social, environmental and economic benefits to Islington.The paper introduces the holistic SLES design approach developed by the GreenSCIES consortium, building upon an initial feasibility study previously published by the authors. The design methodology described takes technical and commercial aspects of a SLES design into account, whilst also explaining the importance of effective stakeholder engagement and co-design with local communities. The paper also provides a technical overview around the intended operation of ambient loop and heat pumps alongside long and short term thermal energy storage. A technical approach selected for integrating electric vehicles (EVs) and solar photovoltaic (PV) is also discussed in detail, alongside the control system architecture developed for the integrated SLES. The paper subsequently moves into demonstrating the benefits of the integrated SLES through a focused scheme design called New River, demonstrating integrated SLES performance compared to conventional systems in a real setting, through a comprehensive energy model. The results presented from a techno-economic analysis demonstrate that significant carbon savings and an attractive internal rate of return of (10%) can be achieved. The results presented show that even the smaller constructible “New River” scheme will save more 5000 tons of CO2e annually. This is a reduction in carbon emissions by 80% over conventional systems and, therefore a major decarbonisation solution in large cities across the world. The GreenSCIES approach presented in this paper is replicable worldwide and could become a central part of delivering nations’ net zero carbon strategy.

A novel LCSA-Machine learning based optimization model for sustainable building design-A case study of energy storage systems

Life Cycle Sustainability Assessment (LCSA) in the built environment has increasingly drawn researchers’ attention in the preceding years; however, the lack of an integrated LCSA based model is still a barrier for its effective implementation into the building design process.This paper proposes a new comprehensive LCSA model to be integrated into the design process of new buildings and energy refurbishment scenarios of existing ones. The proposed model consists of sets of mathematic equations to describe LCSA pillars of buildings, including Life Cycle Assessment (LCA), Life Cycle Costing (LCC), and Social Life Cycle Assessment (SLCA) as the intermediate indices. A final LCSA index is then provided to describe a design scenario's performance from an LCSA perspective using the authors' new formulation and weighting method. The model is also integrated into the optimization process and enhanced with Machine Learning (ML) methods to accelerate the design-assessment process while preserving its accuracy. Finally, given the transition towards buildings' electrification by Renewable Energy Sources (RESs), as a supportive technology, the size-optimization of a residential building's short-term thermal and electrical Energy Storage Systems (ESSs) is chosen to demonstrate the model's capabilities. For this purpose, the building case study is parametrically modeled in Grasshopper, Energy plus, and Matlab for energy analysis, data processing, and machine learning.

Effect of phase change materials on the short-term thermal storage in the solar receiver of dish-micro gas turbine systems: A numerical analysis

• Solar receiver with short-term thermal storage for Dish-Micro Gas Turbine system. • Effect of suitable PCMs on the output parameters of PCM integrated solar receiver. • Detailed numerical analysis using CFD methods. • Steady state parametric analysis for the effect of important parameters of PCM. • Detailed transient analysis for thermal storage behavior of the solar receiver. The solar dish-micro gas turbine system is a new alternative to the existing technologies for small-scale power production (<100 kW) in rural off-grid areas. In these solar-only systems, the short-term thermal energy storage in the solar receiver reduces the effect of natural fluctuations of the solar flux and ensures the stable working fluid (WF) temperature to the gas turbine. It improves the system performance and stability of the small gas turbine. In this context, a concept of Phase Change Material (PCM) integrated solar receiver was presented by authors in previous studies, where the receiver was filled with high-temperature PCM and WF (air) tubes were immersed in it. The PCM stores heat during charging phase and releases heat during the solar breakdown. The selection of PCMs for this receiver is a challenging task due to the high operating temperature ( T m > 1000 K). This study deals with the analysis of the impact of main parameters of the PCM on the performance of the solar receiver and detailed thermal analysis of the component using selected suitable PCMs. The 3D numerical analysis has been performed using Ansys Fluent R2. Moreover, the optical analysis has been carried out by ray tracing using SolTrace. The results of Soltrace are input to the Ansys Fluent and solar radiations are made to impinged on the cavity of the solar receiver. Steady-state parametric analysis has been performed considering the effect of latent heat of fusion, thermal conductivity, and the specific heat of the PCM on the outlet parameters of the receiver. The results showed a significant effect of these parameters on the heat storage of the solar receiver and the solidification/melting rate of PCM. The latent heat has major effect on the thermal storage in the PCM while thermal conductivity has high impact on the solidification/melting rate of the PCM. The transient simulations have been performed for the detailed melting/solidification behavior of the receiver using four metallic PCMs including MgSi, AlSb, NiSi, and Mg 2 Si. AlSb is characterized with comparatively lower latent heat and high thermal conductivity and showed quick solidification in discharging phase. Mg 2 Si has high melting temperature and a low thermal conductivity as compared to the other selected PCMs and thus has a comparatively slow solidification rate than AlSb and MgSi. The results show a good behavior of the component short term (15–20 min) thermal energy storage and stabilization of WF (air) temperature above 1000 K during this period.

Optimal design and energy management of a renewable energy plant with seasonal energy storage

The exploitation of fossil fuels is undoubtedly responsible of environmental problems such as global warming and sea level rise. Unlike energy plants based on fossil fuels, energy plants based on renewable energy sources may be sustainable and reduce greenhouse gas emissions. However, they are unpredictable because of the intermittent nature of environmental conditions. For this reason, energy storage technologies are needed to meet peak energy demands thanks to the stored energy. Moreover, the renewable energy systems composing the plant must be optimally designed and operated. Therefore, this paper investigates the challenge of the optimal design and energy management of a grid connected renewable energy plant composed of a solar thermal collector, photovoltaic system, ground source heat pump, battery, one short-term thermal energy storage and one seasonal thermal energy storage. To this aim, this paper develops a methodology based on a genetic algorithm that optimally designs a 100% renewable energy plant with the aim of minimizing the electrical energy taken from the grid. The load profiles of thermal, cooling and electrical energy during a whole year are taken into account for the case study of the Campus of the University of Parma (Italy).

Experimental investigation on thermal performance of cavity receiver integrated with short-term thermal energy storage for a solar parabolic dish concentrator

Solar parabolic dish collectors are widely used for solar thermal energy conversion for various medium- and high-temperature applications. One of the problems being faced by the receivers of parabolic dish collectors is the short-time non-availability of solar radiation because of cloud cover. This affects the useful output and also results in a sudden change in the receiver temperature, which induces thermal stress in it. Hence receiver with short-term thermal energy storage is attempted to tackle this issue. A hemispherical cavity receiver is developed with a double-layered wall, and the space between the two layers is filled with a salt bath composed of a eutectic mixture of NaNO3 and KNO3 as phase change material. The heat transfer fluid flows through the copper tubes that are coiled in the gap between the two layers. The phase change material surrounds the copper tubes and acts as a storage as well as a heat transfer medium. Experimental results show that the integration of the storage medium to the receiver has a good impact on the thermal output, even during irregular radiation input conditions. The receiver has attained a net energy efficiency of 31.4% for an average irradiance of 561 Wm−2.

Laboratory study on the thermal performance of vertical U‐tube ground heat exchanger during short‐term borehole thermal energy storage ( BTES ) and heat extraction process

To explore the thermal performance of vertical U-tube ground heat exchanger (GHE) during short-term thermal energy storage (TES) and heat extraction process, a novel and well-controlled sandbox with a plate heat exchanger at the bottom of GHE was proposed and established. According to the experiments of short-term TES and heat extraction under different inlet temperature with Re between 2677 and 2823, we found that the thermal performance of GHE was mainly determined by the dynamic soil temperature distribution around GHE, which was induced by the inlet temperature and operation process of GHE. Furthermore, the energy was concentrated within a limited radius (0.4 m) around the GHE during the 8 hours TES and heat extraction process, which was of benefit to the efficient operation of GHE and the thermal environment of soil. Moreover, compared to the simple extraction process C10 (−/8°C) without TES, the heat extraction process of C5 (35/8°C) obtained a 47.8% increase of average heat extraction rate. In addition, approximately 35% of the injected heat was left in the experimental soil under thermal balance conditions C3, C5, and C7. These results show the superiority and characteristic of short-term TES and heat extraction for GHE operation.

High-temperature phase change materials for short-term thermal energy storage in the solar receiver: Selection and analysis

Abstract The Dish-micro gas turbine (D-MGT) system could be an alternative way for small-scale power production ( D -MGT system, preliminary analysis of the receiver with the selected PCMs and the detailed thermal analysis using a numerical approach. After the detailed survey, metallic PCMs are found suitable for this particular application. Based on the requirements of the MGT, the initial collection of the PCMs has melting temperatures higher than 800 ⁰C and the latent heat of fusion greater than 500 kJ/kg. Four PCMs have been shortlisted based on their suitable properties for this application and selected for further analysis. The melting behaviour of the PCMs has modelled analytically by considering one dimensional Stefan problem. The effect of the properties of the PCM on its melting has been investigated by analytical models. The detailed charging and discharging behavior of the PCMs has been carried out with the help of 3D CFD simulations using Ansys Fluent 19.1. The results showed the good behavior of PCMs in stabilizing the outlet air temperature above 800 ⁰C for 20–30 min of discharge phase. The MgSi showed better results than other candidates as it could maintain the outlet air temperature above 850 °C for 30 min during the discharging phase and completely melt after the 90 min of charging phase.

Perspectives for short-term thermal energy storage using salt hydrates for building heating

In this forward-looking perspective, the current research status of latent heat storage using salt hydrates for building heating are firstly analyzed from aspects of material development, performance evaluation, heat transfer enhancement and application feasibility. Based on the analysis, barriers for the further promotion of this technology, including narrow application range, imperfect performance evaluation, inefficient heat transfer enhancement and vague market prospect are outlined. To address these issues, perspectives on four aspects are provided. First, further explorations on salt hydrates with high melting points meeting the heat dispatching demand of centralized building heating are strongly recommended. Second, effects of supercooling and phase separation of salt hydrates in practical applications should be considered differently from lab-scale experiments. Third, the combination of multiple heat transfer enhancement approaches can further improve the overall performance of heat storage. Fourth, this technology has a certain prospect in high-temperature heat dispatching in densely populated areas and short-distance mobile heat supply for emergency. The above perspectives provide guidelines for the future material development, device design, system optimization, and application scenario selection of latent heat storage using salt hydrates for building heating.

Design of high-temperature solar receiver integrated with short-term thermal storage for Dish-Micro Gas Turbine systems

Small-scale concentrated solar power plants equipped with Micro Gas Turbines (MGTs) could be an effective way for power production in the off-grid areas. One of the most relevant concerns for such no fuel-assisted systems is the solar radiation fluctuations, which reduce system performance and, in the worst cases, can damage the MGT. To overcome this issue, the solar receiver should have sufficient thermal inertia averaging the thermal power inputted in the MGT. In the present study, the design of a novel high-temperature cavity solar receiver is presented for the application in the Dish-MGT system. The receiver has been integrated with a PCM for the short-term thermal energy storage (15–30 min). The design process was based on the main parameters of the solar dish and MGT. The thermal model enables the variation of different geometrical factors and input parameters to identify the optimum receiver design. Moreover, it can be used for the evaluation of heat losses from the cavity surface and temperature distributions in the PCM. A 1D steady-state model has been developed for the heat loss calculation from the cavity and corresponding cavity surface temperature, as well as for the evaluation of number and dimensions of the MGT working fluid tubes. Furthermore, the PCM melting and the transient temperature distribution inside the PCM domain have been determined analytically with the help of a 1D thermal model. The initial design results showed that solar receiver integrated with the high-temperature PCM is a promising option for the stable outlet temperature for MGT.

Design optimization of the phase change material integrated solar receiver: A numerical parametric study

Dish-Micro Gas Turbine (D-MGT) systems can be an effective way of power production (<100 kW) in rural areas having limited access to electricity. In such no fuel assisted systems, the stability of the input thermal power to the MGT is an important concern as MGT is sensitive to the temperature variations caused by the natural fluctuation of the solar flux. To reduce this effect, a novel solar receiver has been proposed integrated with a Phase Change Material (PCM) for the short-term thermal energy storage. The proposed receiver is cylindrical shaped filled with PCM, with a conical cavity on the front surface and heat transfer fluid tubes immersed in the PCM. This paper deals with the investigation of the optimum design point of the proposed receiver based on the required input parameters of the MGT. The numerical simulations have been conducted using Computational Fluid Dynamic methods to pre-evaluate the effect of many controlling parameters on the temperature, PCM liquid fraction, pressure loss and outlet thermal power of the receiver. Surface-to-Surface (S2S) radiation model has been employed along with the ray-tracing model for the constant concentrated solar flux of 500 kW/m2 on the receiver aperture surface. The influencing factors considered in this study include the high-temperature PCMs, receiver cavity dimensions, incident solar flux, hot wall thickness, tube diameter and the number of the tubes, pressure drop inside the tubes and the inlet mass flow rate. Results demonstrate the considerable effect of each variable on the receiver output parameters, and this leads to the identification of optimum design point. The result of the study offers valuable guidelines for the receiver development for further experimentation.

Optimization of the hydraulic performance and integration of a heat storage in the geothermal and waste-to-energy district heating system of Ferrara

District heating is an effective way to increase the share of renewables in the thermal energy sector. The district heating network (DHN) of the municipality of Ferrara in northern Italy uses a geothermal reservoir at a depth of 1 km supplying hot water at 100 °C. Besides geothermal energy, the overall annual thermal demand of 170 GWh is covered by a combined heat and power (CHP) plant fed by municipal solid waste (MSW), and by auxiliary natural gas boilers. A short-term thermal energy storage having a useful capacity of 45 MWh decouples the heat production plants from the distribution network. An advanced monitoring and control system has been recently implemented to supply a high-level service to the customers connected to the network, searching for the maximum electricity saving by reducing the supply pressure of the pumping station, and targeting the maximum natural gas saving by a proper production programming of the heat supply plants. The paper describes the main design and operational features of the district heating system, and shows how the improved operation was successful in reducing the electricity and natural gas consumption. The new pressure management consists in monitoring the differential pressure in many substations, especially those located at the periphery of the network, and adjusting the water supply pressure in order to fulfil the minimum differential pressure at the most critical substation. The modified production scheduling consists in keeping using the geothermal energy and waste-to-energy plants in periods of low thermal demand and low electricity prices to daily charge the storage system. Experimental data measured in the first months of improved operation show a high saving potential of electric power for pumping equal to 400 MWh/year, which represents a saving of 30% compared to the previous “manual” optimization of the hydraulic performance. Moreover, the enhanced storage operation enables a saving of 7000 MWh/year of energy from natural gas, which finally contributes for only 11% to the annual heat demand. The calculated payback time of only a few years shows the high cost saving potential of these energy conservation measures in such systems.