Geothermal based hybrid energy systems, toward eco-friendly energy approaches

Geothermal Energy is a very attractive source of naturally-occurring green renewable energy. Exploiting this natural resource is straightforward and causes almost no ill effects to the environment. But, while geothermal does not suffer the intermittence of other renewable sources, its extraction efficiency is fairly modest as compared to other sources. As a result, there has been significant interest recently in hybrid systems that integrate geothermal and other forms of energy to increase the output efficiency. This work will survey the different possible integrations involving geothermal energy. A review of the literature shows that the most common hybrid systems implementation involve the integration of geothermal with solar (45% of systems) followed by the integration of a cooling tower into the geothermal system (30% of systems). This work will also investigate the applications for geothermal hybrids and show that 44% of systems are designed for heating applications. Another 44% are used for cooling while only 12% are designed for electrical power generation. Complexity of control remains as the main obstacle facing hybrid multi-source energy systems including those involving geothermal energy

Enabling technologies for sector coupling: A review on the role of heat pumps and thermal energy storage

In order to reduce greenhouse gas emissions, current and future energy systems need to be made more efficient and sustainable. This change can be accomplished by increasing the penetration of renewable energy sources and using efficient technologies in energy generation systems. One way to improve the operation of the whole energy system is through the generation and end-use sector coupling. Power-to-heat energy conversion and storage technologies, in this view, are enabling technologies that can help in balancing and improving the efficiency of both thermal and electric grids. In the present paper, a comprehensive analysis of the role of heat pumps and thermal energy storage for sector coupling is presented. The main features of the analyzed technologies are presented in the context of smart electric grid, district heating and cooling and multi-carrier energy systems, and recent findings and developments are highlighted. Finally, the technical, social, and economic challenges in the adoption of investigated technologies are discussed.This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31—MCIU/AEI/FEDER, UE) and by the Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación (AEI) (RED2018-102431-T). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to her research group GREiA (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia program

Large-scale solar district heating plants in Danish smart thermal grid: developments and recent trends

Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favorable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70% of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark

A Practical Approach to Model Predictive Control (MPC)for Solar Communities

RÉSUMÉ Les réseaux de chaleur solaire (SDH pour Solar District Heating) font partie des solutions pour réduire la consommation d'énergie et les émissions de Gaz à Effet de Serre (GES) dues aux besoins de chauffage. Ce type d'installation permet de profiter des effets d’économie d’échelle et des avantages d'avoir un système centralisé qui facilite l’intégration de l'énergie solaire pour réduire la dépendance aux carburants fossiles. Un système SDH est un concept éprouvé qui peut être complémenté avec l'ajout de stockage à long terme de l'énergie thermique pour compenser le décalage dans le temps entre l'offre d'énergie solaire et la demande de la charge de chauffage. Ces systèmes sont surtout déployés en Europe; au Canada, la seule installation de SDH est la communauté solaire Drake Landing (DLSC pour Drake Landing Solar Community). Ce projet, qui comprend du stockage saisonnier (BTES pour Borehole Thermal Energy Storage), a été un grand succès, il a atteint 95% de fraction solaire à la cinquième année d'opération. Un système SDH ne peut être complet sans un système de commande qui coordonne le fonctionnement et l'interaction des composants de l’installation. Le contrôle est basé sur un ensemble de règles qui prennent en considération l’état interne du système et les conditions extérieures pour garantir le confort des occupants avec un minimum de consommation de combustibles fossiles. Ce projet de recherche se concentre principalement sur la conception et l'évaluation des nouveaux mécanismes de commande visant à l'augmentation de l'efficacité énergétique globale des systèmes SDH. L'étude de cas est le projet DLSC, et les stratégies de commande proposées sont basées sur l'application pratique des concepts de la Commande Prédictive basée sur des Modèles (MPC pour Model Predictive Control). Un modèle calibré de DLSC qui inclut les stratégies de commande a été développé dans TRNSYS, en s'appuyant sur le modèle utilisé pour les études de conception. Le modèle a été amélioré et de nouveaux composants ont été créés. Le processus de calibration a montré un très bon accord pour les indices annuels de performance énergétique (2% pour la consommation de gaz et pour la partie solaire de l’énergie thermique livrée au réseau de chaleur et, 5% pour la consommation d'électricité).----------ABSTRACT Solar district heating (SDH) systems are part of the solution to reduce energy consumption and GHG emissions required for space heating. This kind of installation takes advantage of the convenience of a centralized system and of solar energy to reduce dependency on fossil-fuels. An SDH system is a proven concept that can be enhanced with the addition of long-term thermal energy storage to compensate the seasonal disparity between solar energy supply and heating load demand. These systems are especially deployed in Europe. In Canada, the only SDH installation is the Drake Landing Solar Community (DLSC). This project, which includes seasonal storage (Borehole Thermal Energy Storage-BTES), has been a remarkable success, reaching a solar fraction of 97% by the fifth year of operation. An SDH system cannot be complete without an appropriate supervisory control that coordinates the operation and interaction of system components. The control is based on a set of rules that must consider the system’s internal status and external conditions to guarantee occupant comfort with minimal fossil-fuels consumption. This research project is mainly focused on conceiving and assessing new control mechanisms aiming towards an increase of SDH systems' overall energy efficiency. The case study is the DLSC plant, and the proposed control strategies are based on the practical application of Model Predictive Control (MPC) theory. A calibrated model of DLSC including the supervisory control strategies was developed in TRNSYS, building upon the model used for design studies. The model was improved and new components were created when needed. The calibration process delivered a very good agreement for the most important yearly energy performance indices (2 % for solar heat input to the district and for gas consumption, and 5 % for electricity use). Proposed control strategies were conceived for modifying four aspects of the current control: the parameters that define the interaction between the Short-Term Thermal Storage (STTS) and the BTES have been optimized so the STTS keeps a higher level of charge in winter-mode operation; a second control strategy forces the BTES discharge when anticipated weather conditions indicate a high heating load and/or reduced solar irradiation; the last two strategies target electricity consumption in the solar loop and the BTES loop by modulating the pumps speeds. Results show that energy efficiency when these control strategies are applied altogether can be improved by about 5% when using perfect forecasts as model’s input

Efficient energy storage in residential buildings integrated with RESHeat system

The Renewable Energy System for Residential Building Heating and Electricity Production (RESHeat) system has been realized for heating and cooling residential buildings. The main components of the RESHeat system are a heat pump, photovoltaic modules, sun-tracking solar collectors and photovoltaic/thermal modules, an under-ground thermal energy storage unit, and a ground heat exchanger. One of the main novelties of the RESHeat system is efficient ground regeneration due to the underground energy storage unit. During a heating season, a large amount of heat is taken from the ground. The underground energy storage unit allows the restoration of ground heating capability and the heat pump's coefficient of performance (COP) to be kept high as possible for consecutive years. The paper presents an energy analysis for a residential building that is a RESHeat system demo site, along with integrating the RESHeat system with the building. The experimentally validated components coupled with the building model to achieve the system performance in TRNSYS software. The results show that the yearly average COP of the heat pump is 4.85 due to the underground energy storage unit. The RESHeat system is able to fully cover the heating demand of the building using renewable energy sources and an efficient underground energy storage system

Control-oriented modelling and operational optimization of a borehole thermal energy storage

Seasonal thermal energy storage is an effective measure to enable a low carbon future through the integration of renewables into the energy system. Borehole thermal energy storage (BTES) provides a solution for long-term thermal energy storage and its operational optimization is crucial for fully exploiting its potential. This paper presents a novel linearized control-oriented model of a BTES, describing the storage temperature dynamics under varying operating conditions, such as inlet temperature, mass-flow rate and borehole connection layouts (e.g. in-series, in-parallel or mixed). It supports an optimization framework, which was employed to determine the best operating conditions for a heat pump-driven BTES, subject to different  intensity profiles of the electricity. It was demonstrated that this boundary condition, due to its seasonal variation, is critical for the optimal operation of the system, as increasing heat pump efficiency in winter while accepting a lower one in summer can be beneficial. Results for an exemplary district case, subject to two different  intensity profiles, show that a lower relative intensity in summer compared to the one in winter leads to a higher optimal operating temperature of the storage. The district system studied is heating-dominated, effectively enabling the BTES to cover only 20% of the total heat demand, leading to limited total yearly CO2 emissions savings of 2.2% to 4.3%. When calculating the benefits associated with the heating and cooling demand handled by the BTES, a higher  emission reduction in the range of 12.8%–19.9% was found. This highlights the BTES potential when subject to more balanced loads.