Higher Primary Energy Savings Research Articles

Influence of the control strategy on the performance of hybrid polygeneration energy system using a prescient model predictive control

Hybrid Polygeneration Energy Systems (HPES) can be effective solutions to reach COP26 goals. In particular, Combined Heat and Power (CHP) systems can increase the total system efficiency when both electrical and thermal power are required. The integration of a Battery Energy Storage System (BESS) can further improve the plant efficiency and economy, assuring higher operational flexibility. Configuration, sizing and control strategy definition are of primary concern for these systems when the best possible performances are sought. This work aims to quantitatively assess the importance of the adopted control strategy in the operation and performance of a possible sub-system of a grid-connected Hybrid Polygeneration Energy Systems (HPES), consisting in this study of a CHP plant assisted by a BESS. A simulation code of the plant from an energetic point of view was used, and the main economic indicator was also calculated according to the legislative reference scenario. Then a Prescient Model Predictive Control (MPC) was coded to achieve near-optimal plant operation, using a novel system state description to reduce the computational burden linked to the Optimal Control Problem (OCP) solution. The BESS system was modelled, including cycle battery ageing. Subsequently, the performances of the adopted prescient MPC have been compared to the previous results given by a multi-objective plant sizing with a rule-based control. The results show that the control strategy can enhance the performance of the CHP system, achieving remarkable overall better performances, with up to 12% higher Primary Energy Savings. Moreover, the research findings highlight how the proposed control variable ensure a reduction of the computational time by more than 70%, also improving the quality of the found solutions to the OCP. The results also suggest that proper control strategies should be adopted even in a preliminary optimal sizing phase of the CHP plant, since there is a large room of improvement in predicting the achievable plant performance when more basic rule-based control strategies are overcome and replaced by MPC.

A techno-enviro-economic assessment of a biomass fuelled micro-CCHP driven by a hybrid Stirling and ORC engine

Stirling engines (SE) offer good part load performance and high heat sink temperatures which make it a suitable candidate to serve as a prime mover in micro-combined cooling, heating and power (μ-CCHP) applications. In this study, a novel μ-CCHP configuration hybridising a SE prime mover with an ORC to utilise the waste heat from the SE to produce additional power is proposed. Additional waste heat was recovered from the flue gas to dry the biomass feedstock, fire a thermal chiller and produce hot water. Further, a non-ideal thermal model was formulated and implemented in MATLAB to model the SE prime mover while the models of the other subsystems were implemented in Aspen plus®. Also, the control of the subsystems of the μ-CCHP was achieved in MATLAB by establishing a connection between the software and Aspen plus®. A detailed sensitivity analysis was conducted to study the influence of cooling and heating loads, rotational speed of the prime mover and quality of the biomass fuel on the energy utilisation factor, primary energy savings (PES), CO2 emissions reduction (CO2ER) and exergy efficiency of the μ-CCHP system. It was found that hybridising SE and ORC increased the power output and thermal efficiency of the standalone SE by 66% and 63.4%, respectively at its operating speed of 2500 rpm, and also improved the performance at high rotational speeds. Further, the deployment of hybrid prime movers in the design of the μ-CCHP yielded high PES and CO2ER of 55% and 43%, respectively when the system utilised woodchips fuel containing 10% moisture. The proposed energy system performs better than conventional energy systems producing only one energy vector over a wide range of engine frequencies, cooling ratios and woodchips compositions.

System design and feasibility of trigeneration systems with hybrid photovoltaic-thermal (PVT) collectors for zero energy office buildings in different climates

Zero or plus energy office buildings must have very high building standards and require highly efficient energy supply systems due to space limitations for renewable installations. Conventional solar cooling systems use photovoltaic electricity or thermal energy to run either a compression-cooling machine or an absorption-cooling machine in order to produce cooling energy during daytime, while they use electricity from the grid for the nightly cooling energy demand. With a hybrid photovoltaic-thermal collector, electricity as well as thermal energy can be produced at the same time. These collectors can produce also cooling energy at nighttime by long-wave radiation exchange with the night sky and convection losses to the ambient air. Such a renewable trigeneration system offers new fields of applications. However, the technical, ecological and economical aspects of such systems are still largely unexplored.In this work, the potential of a PVT system to heat and cool office buildings in three different climate zones is investigated. In the investigated system, PVT collectors act as a heat source and heat sink for a reversible heat pump. Due to the reduced electricity consumption (from the grid) for heat rejection, the overall efficiency and economics improve compared to a conventional solar cooling system using a reversible air-to-water heat pump as heat and cold source.A parametric simulation study was carried out to evaluate the system design with different PVT surface areas and storage tank volumes to optimize the system for three different climate zones and for two different building standards. It is shown such a systems are technically feasible today. With a maximum utilization of PV electricity for heating, ventilation, air conditioning and other electricity demand such as lighting and plug loads, high solar fractions and primary energy savings can be achieved.Annual costs for such a system are comparable to conventional solar thermal and solar electrical cooling systems. Nevertheless, the economic feasibility strongly depends on country specific energy prices and energy policy. However, even in countries without compensation schemes for energy produced by renewables, this system can still be economically viable today. It could be shown, that a specific system dimensioning can be found at each of the investigated locations worldwide for a valuable economic and ecological operation of an office building with PVT technologies in different system designs.

Evaluation of Excess Heat Utilization in District Heating Systems by Implementing Levelized Cost of Excess Heat

District heating plays a key role in achieving high primary energy savings and the reduction of the overall environmental impact of the energy sector. This was recently recognized by the European Commission, which emphasizes the importance of these systems, especially when integrated with renewable energy sources, like solar, biomass, geothermal, etc. On the other hand, high amounts of heat are currently being wasted in the industry sector, which causes low energy efficiency of these processes. This excess heat can be utilized and transported to the final customer by a distribution network. The main goal of this research was to calculate the potential for excess heat utilization in district heating systems by implementing the levelized cost of excess heat method. Additionally, this paper proves the economic and environmental benefits of switching from individual heating solutions to a district heating system. This was done by using the QGIS software. The variation of different relevant parameters was taken into account in the sensitivity analysis. Therefore, the final result was the determination of the maximum potential distance of the excess heat source from the demand, for different available heat supplies, costs of pipes, and excess heat prices.

Configuration based modeling and performance analysis of single effect solar absorption cooling system in TRNSYS

Solar based cooling systems may assume various configurations depending upon the manner in which the individual system components are inter-connected and the adopted control scheme. In this study, simulation based performance analysis of a solar assisted single effect absorption cooling system is done for two system configurations in TRNSYS. The analysis is carried out to meet a peak cooling demand of 298 kW for an educational building located in Islamabad (33.71°N, 73.06°E). In configuration-1 (C-1), the working fluid returning from the generator of the absorption chiller always flows towards the hot storage tank which acts as a common element between the solar collector and the absorption chiller. In configuration-2 (C-2), the working fluid returning from the absorption chiller may become isolated from the collector-storage tank loop if the fluid temperature in the storage tank is less than the required temperature (i.e. 110 °C) and is directly fed to the auxiliary boiler. Both system configurations are fully modelled in TRNSYS and dynamic simulations are run for the entire summer season. Various performance factors such as solar fraction, collector efficiency and primary energy savings are evaluated to optimize the key system design variables which include collector tilt, storage volume, type and size of the solar collector. Simulation results demonstrate that for the whole summer season, C-2 with flat plate or evacuated tube solar collectors always results in higher primary energy savings than C-1. However, on the basis of solar fraction and monthly collector efficiency, difference between C-1 and C-2 is estimated to be marginal. Overall, C-2 in conjunction with evacuated tube collectors (ETC) results in minimum collector area per kilowatt of cooling demand and higher monthly collector efficiency than C-1.

Energy analysis of swimming pools for sports activities: cost effective solutions for efficiency improvement

Swimming pools for sports activities are very much energy-consuming since, in addition to the energy requirements that are common to all types of sports facilities (air conditioning and lighting in large spaces, high levels of water heating requirements, etc.), heating, filtration and continuous replacement of water must be considered, which involve huge consumption of natural resources in terms of primary non-renewable energy resources (according to the local fossil/renewable mix) and drinking water, whose use is mandatory. The paper calculates, through our ad-hoc developed algorithm (named EnerPool), potential savings in terms of non-renewable primary energy consumption, achievable through energy efficiency actions involving heating, filtration and water replacement. This work analyses in detail a number of possible solutions to reduce heat needs, therefore allowing high non-renewable primary energy savings (up to more than 50%) at low cost and with a payback time of less than two years. Thanks to the facilities’ data (number and size of pools, utilization rate, etc.) supplied by CONI (the Italian National Olympic Committee) about swimming schools and facilities affiliated to FIN (the Italian Swimming Federation), and based on the results of the analysis on energy efficiency actions, this article presents nationwide estimates of the potential savings of non-renewable primary energy through cost-effective efficiency improvement solutions. Furthermore, the solutions analyzed are not alternative to other heat production solutions through high-efficiency systems (condensing boilers, water/air heat pumps, combined heat and power production plants) or by means of renewable sources (solar collectors, photovoltaic panels), therefore achieving economic and energy savings albeit with much higher initial costs.

Energy Performance and Thermo-economic Assessment of a Microturbine-based Dual-fuel Gas-biomass Trigeneration System

Abstract The focus of this paper is on the energy performance and thermo-economic assessment of a small scale (100 kWe) combined cooling, heat and power (CCHP) plant serving a tertiary/residential energy demand fired by natural gas and solid biomass. The plant is based on a modified regenerative micro gas-turbine (MGT), where compressed air exiting the recuperator is externally heated by the hot gases produced in a biomass furnace. The flue gases after the recuperator flow through a heat recovery system (HRS), producing domestic hot water (DHW) at 90 °C, space heating (SH), and also chilled water (CW) by means of an absorption chiller (AC). Different biomass/natural gas ratios and an aggregate of residential end-users in cold, average and mild climate conditions are compared in the thermo-economic assessment, in order to assess the trade-offs between: (i) the lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) the higher primary energy savings and revenues from feed-in tariffs available for biomass electricity exported into the grid; and (iii) the improved energy performance, sales revenue and higher investment and operational costs of trigeneration. The results allow for a comparison of the energy performance and investment profitability of the selected system configuration, as a function of the heating/cooling demand intensity, and report a global energy efficiency in the range of 25-45%, and IRR in the range of 15-20% assuming the Italian subsidy framework.

Externally Fired Micro-Gas Turbine and Organic Rankine Cycle Bottoming Cycle: Optimal Biomass/Natural Gas Combined Heat and Power Generation Configuration for Residential Energy Demand

Small-scale combined heat and power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on-site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small-scale micro-gas turbine for heat and power cogeneration, a bottoming organic Rankine cycle (ORC) system can be coupled to the cycle, however, this option reduces the temperature and the amount of cogenerated heat available to the thermal load. In this perspective, the paper presents the results of a thermo-economic analysis of small-scale CHP plants composed of a micro-gas turbine (MGT) and a bottoming ORC, serving a typical residential energy demand. For the topping cycle, three different configurations are examined: (1) a simple recuperative micro-gas turbine fueled by natural gas (NG); (2) a dual fuel externally fired gas turbine (EFGT) cycle, fueled by biomass and natural gas (50% share of energy input) (DF); and (3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming ORC is a simple saturated cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild, and hot climate conditions) and CHP plant operational strategies (baseload versus heat-driven versus electricity-driven operation mode) on the global energy efficiency and profitability of the following three configurations: (A) MGT with cogeneration; (B) MGT+ ORC without cogeneration; and (C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following trade-offs: (i) lower energy conversion efficiency and higher investment cost of biomass input with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration with the bottoming ORC; and (iii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.

Part Load Performance and Operating Strategies of a Natural Gas—Biomass Dual Fueled Microturbine for Combined Heat and Power Generation

The focus of this paper is on the part load performance of a small scale (100 kWe) combined heat and power (CHP) plant fired by natural gas (NG) and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative microgas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with NG takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for preheating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant. Different biomass/NG energy input ratios are also modeled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate and (ii) higher primary energy savings (PESs) and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD), and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average, and mild climate conditions.

Energy Efficient Integration of Heat Pumps into Solar District Heating Systems with Seasonal Thermal Energy Storage

Solar district heating (SDH) with seasonal thermal energy storage (STES) is a technology to provide heat for space heating and domestic hot water preparation with a high fraction of renewable energy. In order to improve the efficiency of such systems heat pumps can be integrated. By preliminary studies it was discovered, that the integration of a heat pump does not always lead to improvements from an overall energy perspective, although the operation of the heat pump increases the efficiency of other components of the system e. g. the STES or the solar collectors. Thus the integration of heat pumps in SDH systems was investigated in detail.Usually, the heat pumps are integrated in such a way, that the STES is used as low temperature heat source. No other heat sources from the ambience are used and only that amount of energy consumed by the heat pump is additionally fed into the system. In the case of an electric driven heat pump, this is highly questionable concerning economic and CO2-emission aspects. Despite that fact the operation of the heat pump influences positively the performance of other components in the system e. g. the STES and makes them more efficient. If the primary energy consumption of the heat pump is lower than the energetic benefits of all other components, the integration makes sense from an energetic point of view.A detailed assessment has been carried out to evaluate the most promising system configurations for the integration of a heat pump. Based on this approach a system concept was developed in which the integration of the heat pump is energetically further improved compared to realised systems. By means of transient system simulations this concept was optimised with regard to the primary energy consumption. A parameter study of this new concept has been performed to identify the most sensitive parameters of the system.The main result and conclusion are that higher solar fractions and also higher primary energy savings can be achieved by SDH systems using heat pumps compared systems without heat pumps.

Natural gas–biomass dual fuelled microturbines: Comparison of operating strategies in the Italian residential sector

This paper compares different operating strategies for small scale (100 kWe) combined heat and power (CHP) plants fired by natural gas and solid biomass to serve a residential energy demand. The focus is on a dual fuel micro gas turbine (MGT) cycle. Various biomass/natural gas energy input ratios are modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions. On the basis of the results from thermodynamic assessment and simulation at partial load operation, CAPEX and OPEX estimates, and Italian energy policy scenario (incentives available for biomass electricity, on-site and high efficiency CHP), the maximum global energy efficiency, primary energy savings and investment profitability is found, as a function of biomass/natural gas ratio, plant operating strategy and energy demand typology. The thermal and electric conversion efficiency ranged respectively between 46 and 38% and 30 and 19% for the natural gas and biomass fired case studies. The IRR of the investment was highly influenced by the load/CHP thermal power ratio and by the operation mode. The availability of high heat demand levels was also a key factor, to avoid wasted cogenerated heat and maximize CHP sales revenues. BL operation presented the highest profitability because of the higher revenues from electricity sales. Climate area was another important factor, mainly in case of low load/CHP ratios. Moreover, at low load/CHP power ratio and for the BL operation mode, the dual fuel option presented the highest profitability. This is due to the lower cost of biomass fuel in comparison to natural gas and the high subsidies available for biomass electricity by feed-in tariffs. The results show that dual fuel MT can be an interesting option to increase efficiencies, flexibility and plant reliability at low cost in comparison to only biomass systems, facilitating an integration of renewable and fossil fuel systems.

Thermo-economic assessment of externally fired micro-gas turbine fired by natural gas and biomass: Applications in Italy

This paper proposes a thermo-economic assessment of small scale (100kWe) combined heat and power (CHP) plants fired by natural gas and solid biomass. The focus is on dual fuel gas turbine cycle, where compressed air is heated in a high temperature heat exchanger (HTHE) using the hot gases produced in a biomass furnace, before entering the gas combustion chamber. The hot air expands in the turbine and then feeds the internal pre-heater recuperator, Various biomass/natural gas energy input ratios are modeled, ranging from 100% natural gas to 100% biomass. The research assesses the trade-offs between: (i) lower energy conversion efficiency and higher investment cost of high biomass input rate and (ii) higher primary energy savings and revenues from bio-electricity feed-in tariff in case of high biomass input rate. The influence of fuel mix and biomass furnace temperature on energy conversion efficiencies, primary energy savings and profitability of investments is assessed. The scenarios of industrial vs. tertiary heat demand and baseload vs. heat driven plant operation are also compared. On the basis of the incentives available in Italy for biomass electricity and for high efficiency cogeneration (HEC), the maximum investment profitability is achieved for 70% input biomass percentage. The main barriers of these embedded cogeneration systems in Italy are also discussed.

Solar heating and cooling systems by CPVT and ET solar collectors: A novel transient simulation model

In this paper, a novel purposely designed dynamic simulation model for the performance analysis of solar heating and cooling systems is presented. The investigated system layouts are based on single stage LiBr–H2O absorption chillers and on both evacuated tube and concentrating photovoltaic thermal solar collectors. Furthermore, both electric chiller and gas fired heater backup system are considered. Such model is implemented in a computer code written in MATLAB. Here, the optimisation of several system design and operating parameters in terms of energy saving is also carried out. A code to code analysis is performed comparing the obtained simulation results vs. those achieved by a TRNSYS model available in literature. The simulation code for the concentrating photovoltaic thermal solar collectors is validated by experimental data. A good agreement among results is observed in both the cases. A suitable case study referred to a building including both offices and dwellings located in Northern and Southern Italy is presented. High primary energy savings are obtained for some of the investigated system layouts. By evacuated tube collectors solar field such savings can reach 74%, while shifting to concentrating photovoltaic thermal solar collectors they often surpass 100%. The system economic profitability strongly depends on future scale economies and eventual public funding.

Simulation and energy efficiency analysis of desiccant wheel systems for drying processes

In drying processes it is necessary to appropriately control air humidity and temperature in order to enhance water evaporation from product surface. The aim of this work is to investigate several HVAC configurations for product drying based on desiccant wheels, in order to find systems which reach high primary energy savings through the appropriate integration of refrigerating machines, adsorption wheels and cogenerative engines. Simulations are carried out for different values of sensible to latent ambient load ratio and the effect of ambient and outside air conditions is evaluated for each configuration. It is shown that primary energy savings can reach 70–80% compared to the reference technology based on a cooling coil. With respect to works available in literature, the results of this study keep a general approach and they can be used as a simple tool for preliminary assessment in a wide range of applications.

Multi-objective, multi-period optimization of biomass conversion technologies using evolutionary algorithms and mixed integer linear programming (MILP)

The design and operation of energy systems are key issues for matching energy supply and demand. A systematic procedure, including process design and energy integration techniques for sizing and operation optimization of poly-generation technologies is presented in this paper. The integration of biomass resources as well as a simultaneous multi-objective and multi-period optimization, are the novelty of this work. Considering all these concepts in an optimization model makes it difficult to solve. The decomposition approach is used to deal with this complexity.Several options for integrating biomass in the energy system, namely back pressure steam turbines, biomass rankine cycles (BRC), biomass integrated gasification gas engines (BIGGE), biomass integrated gasification gas turbines, production of synthetic natural gas (SNG) and biomass integrated gasification combined cycles (BIGCC), are considered in this paper. The goal is to simultaneously minimize costs and CO2 emission using multi-objective evolutionary algorithms (EMOO) and Mixed Integer Linear Programming (MILP).Finally the proposed model is demonstrated by means of a case study. The results show that the simultaneous production of electricity and heat with biomass and natural gas are reliable upon the established assumptions. Furthermore, higher primary energy savings and CO2 emission reduction, 40%, are obtained through the gradual increase of renewable energy sources as opposed to natural gas usage. However, higher economic profitability, 52%, is achieved with natural gas-based technologies.