Solar seasonal thermal energy storage for space heating in residential buildings: Optimization and comparison with an air-source heat pump

ABSTRACT

This study evaluates the techno-economics of replacing an air-source heat pump (ASHP) system with a solar seasonal thermal energy storage (STES) system for space heating in Hangzhou, China. Three heating systems, solar STES, ASHP, and ASHP with short-term storage of solar energy, are developed using TRNSYS for a house with 240 m² of floor area. The ratio of tank volume to collector area (RVA) of the STES is optimized for the lowest equivalent annual cost over a lifespan of 20 y. The determined optimal RVA is 0.33 m³/m², although it depends on the system and electricity prices. The optimized STES reduces the electricity demand to 1,269 kWh (74% reduction). Despite the superior energy performance, the economic benefit is only possible with large STES systems, which enjoy low tank prices due to scale effects. The results suggest that policy support is needed for STES, where district scaling is not an option.     

Review on solar air heating system with and without thermal energy storage system

In order to produce process heat for drying of agricultural, textile, marine products, heating of buildings and re-generating dehumidify agent, solar energy is one of the promising heat sources for meeting energy demand without putting adverse impact of environment. Hence it plays a key role for sustainable development. Solar energy is intermittent in nature and time dependent energy source. Owing to this nature, PCMs based thermal energy storage system can achieve the more popularity for solar energy based heating systems. The recent researches focused on the phase change materials (PCMs), as latent heat storage is more efficient than sensible heat storage. In this paper an attempt has been made to present holistic view of available solar air heater for different applications and their performance.     

Investigation of thermal performance of a ground coupled heat pump system with a cylindrical energy storage tank

An analytical and computational model for a solar assisted heat pump heating system with an underground seasonal cylindrical storage tank is developed. The heating system consists of flat plate solar collectors, an underground cylindrical storage tank, a heat pump and a house to be heated during winter season. Analytical solution of transient field problem outside the storage tank is obtained by the application of complex finite Fourier transform and finite integral transform techniques. Three expressions for the heat pump, space heat requirement during the winter season and available solar energy are coupled with the solution of the transient temperature field problem. The analytical solution presented can be utilized to determine the annual variation of water temperature in the cylindrical store, transient earth temperature field surrounding the store and annual periodic performance of the heating system. A computer simulation program is developed to evaluate the annual periodic water and earth temperatures and system performance parameters based on the analytical solution. Copyright © 2003 John Wiley & Sons, Ltd.     

Feasibility study of a district energy system with seasonal water thermal storage

In this paper, performance of three types of district heating/cooling and hot water supply system with natural and unused energy utilization were examined by using system simulation. An area zoned for both commercial and residential buildings was chosen for this study. The first system is the conventional system in which an electric driven turbo chiller and a gas-fired boiler are installed as the heat source. This is considered as the reference system. Two alternative systems utilize waste heat from space cooling and heating. One is designed based on short-term heat recovery and the other employs the concept of an annual cycle energy system (i.e. seasonal heat recovery). All of the three systems use solar thermal energy for hot water supply to the residential zone. The index for evaluation is the coefficient of performance of the overall system, based on primary energy. As a result, it was found that the seasonal storage system could decrease the energy consumption by about 26% and the short-term heat recovery system could decrease it by about 16% compared with the reference system. In designing the heat recovery system, a balance of cooling/heating demand is an important factor. Therefore a sensitivity analysis of performance of the overall system and the seasonal thermal storage for several load patterns was performed. From these results, it was found that if the amount of heating/cooling demand were well balanced, an improvement of energy performance could be achieved and the utilization factor of the seasonal tank would become higher. Furthermore, the volume of the seasonal storage tank could be reduced.     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。     

Performance analysis of high-capacity thermal energy storage using solid-gas thermochemical sorption Principle

储能技术是提高能源利用效率的一种有效手段,可有效调配能量供给与需求在时间,空间和强度上的匹配关系,传统显热储存技术和相变潜热储存技术的储热密度一般在100~200 kJ/kg,储热能力较低不利于规模化应用.本工作提出一种基于固-气化学反应的大容量热化学吸附储热方法,利用吸附工质对在化学反应过程中热能与化学吸附势能的相关转化实现热量的储存和释放,具有高效储热的显著优点.采用4种典型温区的吸附储热工质对为例进行了热化学吸附储热热力循环特性及工作性能的理论研究,在此基础上对不同温区吸附储热工质对(50~280 ℃)的热化学物性参数,储热温度,储热密度进行了分析比较,以期实现不同温度品位的热量储存.结果表明:热化学吸附储热技术的反应盐储热密度高达2000 kJ/kg以上,其储能密度约为传统显热储存技术和相变潜热储存技术的10~20倍,是一种具有发展潜力的大容量,高性能储热方法,该新技术可为规模化工业储热应用及太阳能等可再生能源的高效利用提供技术支撑.     

The solar heating system with seasonal storage at the Solar-Campus Jülich

The Solar-Campus Jülich is an area of 14 ha in the north of the existing University of Applied Sciences. Three independent partners are constructing low energy buildings on this site (heating demand 144 MJ m⁻² a⁻¹). To date (June 2000) an auditorium with a library has been completed as well as an additional laboratory building. The Students’ Association Aachen has erected 23 houses with accommodation for 136 students. The houses are arranged in five rows, each of them demonstrating different kinds of modern energy-saving technologies for heating and ventilation. The energy utility in Jülich intends to build industrial buildings and move their complete organisation from southern Jülich to the Solar-Campus. A solar district heating concept, with seasonal storage, is planned to cover about 50–60% of the heating demand of all the buildings. The paper gives details of the design of the pyramidal seasonal storage tank including cost analysis. The top cover of the 2500 m³ tank will be constructed of several insulated floating pontoons, which are connected to each other, so that it is possible to walk on it. The 1200 m² of collectors are distributed over the different buildings and coupled to the underground storage tank by either a 2-, 3- or 4-pipe distribution network. Due to the low energy demand, the overall energy consumption will be low (2124 GJ a⁻¹). The heating cost consequently will be high: 0.17 DM/kWh with a conventional gas-based system, and 0.54 DM/kWh with the solar system including seasonal storage.     

High temperature solar heated seasonal storage system for low temperature heating of buildings

A preliminary study of a solar-heated low-temperature space-heating system with seasonal storage in the ground has been performed. The system performance has been evaluated using the simulation models TRNSYS and MINSUN together with the ground storage module DST. The study implies an economically feasible design for a total annual heat demand of about 2500 MWh. The main objective was to perform a study on Anneberg, a planned residential area of 90 single-family houses with 1080 MWh total heat demand. The suggested heating system with a solar fraction of 60% includes 3000 m² of solar collectors but electrical heaters to produce peak heating. The floor heating system was designed for 30°C supply temperature. The temperature of the seasonal storage unit, a borehole array in crystalline rock of 60,000 m³, varies between 30 and 45°C over the year. The total annual heating costs, which include all costs (including capital, energy, maintenance etc.) associated with the heating system, were investigated for three different systems: solar heating (1000 SEK MWh⁻¹), small-scale district heating (1100 SEK MWh⁻¹) and individual ground-coupled heat pumps (920 SEK MWh⁻¹). The heat loss from the Anneberg storage system was 42% of the collected solar energy. This heat loss would be reduced in a larger storage system, so a case where the size of the proposed solar heating system was enlarged by a factor of three was also investigated. The total annual cost of the solar heating system was reduced by about 20% to about 800 SEK MWh⁻¹, which is lower than the best conventional alternative.     

Energy management in horticultural applications through the closed greenhouse concept,state of the art

The commercial greenhouse has the highest demand for energy as compared to all other agricultural industry sectors. Here, energy management is important from a broad sustainability perspective. This paper presents the state-of-the-art regarding one energy management concept; the closed greenhouse integrated with thermal energy storage (TES) technology. This concept is an innovation for sustainable energy management since it is designed to maximize the utilization of solar energy through seasonal storage. In a fully closed greenhouse, there is no ventilation which means that excess sensible and latent heat must be removed. Then, this heat can be stored using seasonal and/or daily TES technology, and used later in order to satisfy the heating demand of the greenhouse. This assessment shows that closed greenhouse can, in addition to satisfying its own heating demand, also supply the demand for neighboring buildings. Several energy potential studies show that summer excess heat of almost three times the annual heating demand of the greenhouse. However, many studies propose the use of some auxiliary system for peak load. Also, the assessment clearly point out that a combination of seasonal and short-term TES must be further explored to make use of the full potential. Although higher amount of solar energy can be harvested in a fully closed greenhouse, in reality a semi-closed greenhouse concept may be more applicable. There, a large part of the available excess heat will be stored, but the benefits of an integrated forced-ventilation system are introduced in order to use fresh air as a rapid response for primarily humidity control. The main conclusion from this review is that aspects like energy efficiency, environmental benefits and economics must be further examined since this is seldom presented in the literature. Also, a variety of energy management scenarios may be employed depending on the most prioritized aspect.     

Solar absorption cooling in Spain: Perspectives and outcomes from the simulation of recent installations

In spite of the fact that Spain is one of the EU countries with the highest solar resource on annual basis, the huge seasonal variation in solar radiation availability and the relatively short period with heating demand, make it difficult to reach significant contributions of solar energy to the buildings heating energy demand. This compromises the economic viability of big solar collector areas per capita, and introduces technical difficulties for the dissipation of the excess solar energy available in the summer months. On the other hand, in a large part of the Spanish territory, in other to reach adequate comfort conditions in our buildings, the energy demand for cooling is more important or of the same order than the heating demand. Cooling energy demand is now experiencing a fast growing rate as this comfort requirement becomes internalized. Domestic air conditioning equipments based on vapour compression cycles are being used to reach comfort conditions in some of the rooms of buildings that were designed without taking into account cooling requirements. In spite of their so far small contribution to the total building sector energy demand, these equipments are already imposing important constraints on the environment and the electricity distribution system. Solar absorption cooling arises as an interesting alternative, which at the same time allows reaching a higher solar contribution to the heating demand. However, solar cooling installations present several peculiarities with respect to the more known DHW or even heating installations, which require to incorporate a more detailed approach and additional considerations in the design and performance evaluation processes. Besides, some limitations still persist in solar absorption systems, which could make them loose their market potential for the benefit of other solar cooling options. In this paper, we present some conclusions arising from the experience gained in detailed TRNSYS dynamical simulation of some of the first commercial solar heating and cooling installations recently implemented in Spain, and analyse their perspectives in comparison with other solar cooling options.     

Modeling and optimization of a solar assisted heat pump using ice slurry as a latent storage material

This paper presents the modeling and optimization of a solar assisted heat pump using ice slurry. Solar collectors are used as the primary source of thermal energy, with two distinct loops allowing the collectors to operate in series with an ice tank, or a warm water tank. Thermal energy stored in the ice tank is transferred to a warm water distribution tank via a heat pump. First, a new mathematical model of an ice slurry storage tank is presented. Validation of the model with experimental data confirms its ability to predict the ice mass and tank fluid temperatures during the charging and discharging modes of operation. The developed ice tank model is combined with the TRNSYS energy simulation program to formulate a complete model of the proposed heat pump system. This computer model then serves as a base for a mathematical optimization with the objective to minimize the energy use for heating and DHW over a single heating season. Simulated results demonstrate the potential of the optimized system in reducing the heating operating energy use of a high performance home in Montreal, QC.     

太阳能与谷电互补供热系统设计

利用太阳能辅以低谷电加热,采用两个储热罐进行交替轮流集热、供热。当正常集热或低温预热时,两个储热罐的水温低者优先集热循环运行;若两储热罐水温相同,则设定储热罐优先集热循环运行。当正常供热、夜间防冻循环和低温维持运行时,太阳能集热系统优先于低谷电循环运行给储热罐加热。当假日集热或低温保温供热时,同时给两储热罐集热循环运行。供热循环水泵和用热循环水泵的启动运行,以供热时优先,其停止运行以用热系统的回水温度大于或等于取暖温度的最大设定值者优先。以可编程控制器和组态软件技术,实现了系统集热、供热时段的定温、定时、定温差。此互补供热系统可以取代传统的高耗能锅炉,节能率达30%以上。     

不同地区住宅能耗与太阳能吸收式空调系统匹配性分析与模拟研究

为了满足农村住宅清洁用能的需求,多种形式的能源系统逐渐开始应用于广大的农村地区。随着太阳能集热器集热效率的提高,热驱动机组各项性能不断改善,这样有利于太阳能吸收式空调系统在农村地区的应用。为了研究太阳能吸收式空调系统与农村住宅全年能耗的匹配问题,文章首先建立了DeST住宅模型,然后利用TRNSYS软件建立了太阳能吸收式空调系统模型,最后根据模拟结果对国内不同气候区内农村住宅供热季、供冷季的平均热负荷值,以及全年的能耗进行分析。此外,文章还分析了典型日太阳能吸收式空调系统的运行策略与效果。分析结果表明:在无辅助热源的条件下,太阳能集热器的集热温度会大于80℃,满足空调机组的热驱动温度,因此可以作为太阳能吸收式空调系统的的热源;当启动温度为85℃时,空调机组的制冷量可以达到8 kW,性能系数COP为0.733。     

Performance of solar assisted heatpump systems in residential applications

In this experimental study, several solar-assisted heating and cooling configurations have beenconsidered for a basic system comprised of a two-speed heat pump, photovoltaic (PV) arrays, solar thermal collectors, and thermal storage. The objective of the study was to determine the performance of the PV arrays at decreased insolation, the effects of air preheat by solar thermal energy on heat pump operation, and cooling system performance under two different configurations. During the entire operation, the PV arrays converted 4.7 per cent (9.5 MWh) of the incident solar insolation to d.c. power, of which 54.6 per cent was used by the residence. This contributed 23.4 per cent of the total house electrical demand. The remaining 45.4 per cent of the output was fed to the utility, indicating the arrays and the heat pump were not properly sized with each other. Based on results from the winter heating operation, it is shown that for the particular heating system consdered, the best performance is attained when the solar heating is used alone. By using the heat pump as a booster, the remaining available solar energy left in the storage tank can be used with good seasonal performance factor. Summer cooling operation consisted of two sequential cooling configurations. In the first cooling test, the heat pump was operated to either the house or storage when the PV array generation level was greater than the energy demand of the heat pump and associated equipment. When the array output level was less than the cooling system demand, the operating strategy was that of an off-peak cooling operation to chill the water storage. Utilization of chilled water storage was not realized in the first cooling test because of the inherent inefficient design of the Tri-X coil. The capacity at low-speed heat pump operation was too small to effect significant cooling of the water loop; whereas high-speed heat pump operation in attempting to chill water (fan operation absent) caused frosting of the coil. The heat pump was utilized only to maintain chilled water storage in the second cooling test, without heat transfer through the Tri-X coil. Cooling system performance obtained in cooling test 2 using the Ametex exchanger was considerably improved over the test 2 performance with the Tri-X coil.     

A comparison of solar absorption system configurations

Solar thermal driven cooling systems for residential applications are a promising alternative to electric compression chillers, although its market introduction still represents a challenge, mainly due to the higher investment costs. The most common system configuration is an absorption chiller driven by a solar thermal system, backed up by a secondary heating source, normally a gas boiler. Heat storage in the primary (solar) circuit is mandatory to stabilize and extend the operation of the chiller, whereas a cold storage tank is not so common.This paper deals with the selection of the most suitable configuration for residential cooling systems with solar energy. In Spain, where cooling needs are usually higher than heating needs, the interest of a reversible heat pump as auxiliary system and a secondary cooling storage are analyzed.A complete TRNSYS model has been developed to compare a configuration with just hot storage (of typical capacity 40 L/m² of solar collector surface) and a configuration with both, hot and cool storages. The most suitable configuration is very sensible to the solar collector area. As the collector area increases, the advantages of a cool storage vanish. Increasing the collector area tends to increase the temperature of the hot storage, leading to higher thermal losses in both the collector and the tank. When the storage volume is concentrated in one tank, these effects are mitigated. The effect of other variables on the optimal configuration are also analyzed: collector efficiency curve, COP of the absorption chiller, storage size, and temperature set-points of the chillers.     

Potential for large-scale solar collector system to offset carbon-based heating in the Ontario greenhouse sector

In the Ontario greenhouse sector the misalignment of available solar radiation during the summer months and large heating demand during the winter months makes solar thermal collector systems an unviable option without some form of seasonal energy storage. Information obtained from Ontario greenhouse operators has shown that over 20% of annual natural gas usage occurs during the summer months for greenhouse pre-heating prior to sunrise. A transient model of the greenhouse microclimate and indoor conditioning systems is carried out using TRNSYS software and validated with actual natural gas usage data. A large-scale solar thermal collector system is then incorporated and found to reduce the annual heating energy demand by approximately 35%. The inclusion of the collector system correlates to a reduction of about 120 tonnes of CO₂ equivalent emissions per acre of greenhouse per year. System payback period is discussed considering the benefits of a future Ontario carbon tax.     

Thermal Energy Storage and Phase Change Materials: An Overview

The storage of thermal energy in the form of sensible and latent heat has become an important aspect of energy management with the emphasis on efficient use and conservation of the waste heat and solar energy in industry and buildings. Latent heat storage is one of the most efficient ways of storing thermal energy. Solar energy is a renewable energy source that can generate electricity, provide hot water, heat and cool a house, and provide lighting for buildings. Paraffin waxes are cheap and have moderate thermal energy storage density but low thermal conductivity and, hence, require a large surface area. Hydrated salts have a larger energy storage density and a higher thermal conductivity. In response to increasing electrical energy costs and the desire for better lad management, thermal storage technology has recently been developed. The storage of thermal energy in the form of sensible and latent heat has become an important aspect of energy management with the emphasis on the efficient use and conservation of the waste heat and solar energy in the industry and buildings. Thermal storage has been characterized as a kind of thermal battery.     

Central electric thermal storage (ETS) feasibility for residential applications: Part 1. Numerical and experimental study

Detailed models have been developed and integrated into a TRNSYS calculation tool to evaluate the optimal storage capacity of central electric thermal storage (ETS) units for residential applications. The tool uses hourly weather data to establish the energy use profile of residential buildings under different heating system configurations and control strategies. The reliability of the system for each configuration is determined as the percentage of time the heating system meets the heating requirement of the building. The level of reliability is used to evaluate the appropriate capacity of the system. This study investigates the theoretical development and modelling of a multi‐zone building equipped with a central ETS in TRNSYS. The TRNSYS simulation results are then compared and validated against experimental data obtained during the project. Furthermore, the effect of various parameters such as system configuration, charging capacity, control strategies, heat losses from the building, and electric utility time‐of‐use schedules are analysed. Copyright © 2001 John Wiley & Sons, Ltd.     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

A numerical study on heat storage water tank containing phase change materials for air source heat pump systems

构建空气源热泵-相变蓄热水箱供暖系统,通过相变储能技术的合理应用,优化了太阳能、空气热能等非连续能源的供能方式,有效提高了建筑中可再生能源的利用率。相变蓄热系统采用了6 m³的保温水箱作为蓄热容器,选取46#石蜡为主要相变材料,304#不锈钢管为封装材料。建立蓄热系统的三维数学模型,采用有效热熔法对相变材料的焓值进行处理,运用Fluent数值模拟软件,研究相变蓄热系统的蓄放热性能。模拟结果显示,系统的蓄热时间为9.2 h,理想蓄热量为102.4 kW·h,能够单独提供低能耗建筑连续采暖11.1 h。空气源热泵-相变蓄热水箱供暖系统能实现大跨度的间歇供暖,在利用非连续能源供暖领域具有良好的前景。