Scholarly article on topic 'Thermal Performance of New Hybrid Solar Energy-phase Change Storage-floor Radiant Heating System'

Thermal Performance of New Hybrid Solar Energy-phase Change Storage-floor Radiant Heating System Academic research paper on "Earth and related environmental sciences"

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{"solar energy application" / "phase change material thermal storage" / "radiant floor heating system" / "thermal performance"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Ye Zhang, Chao Chen, Hao Jiao, Wanjiang Wang, Zongyi Shao, et al.

Abstract A new solar energy-phase change storage-floor radiant heating system is proposed to provide a comfort indoor environment in winter. In this study the proposed new system is applied in an office building in Urumqi in China. The system consists mainly of three units: evacuate heat pipe solar collectors (as heat collection unit), vertical phase change thermal energy storage device (VPCTESD) and floor radiant heating system. The influence on thermal efficiency of solar collector, inlet and outlet water temperature, connect method during local heating period and thermal performance of this system was developed experimentally. The results reported in this paper adequately support the new system as an alternative to the traditional solar heating system, and it can increase 30% utilization of solar energy than traditional solar heating system. The research result provides a reference for optimization design of this kind of system.

Academic research paper on topic "Thermal Performance of New Hybrid Solar Energy-phase Change Storage-floor Radiant Heating System"

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Procedía Engineering 146 (2016) 89 - 99

Procedía Engineering

www.elsevier.com/locate/procedia

8th International Cold Climate HVAC 2015 Conference, CCHVAC 2015

Thermal performance of new hybrid solar energy-phase change storage-floor radiant heating system

Ye Zhanga*, Chao Chenb, Hao Jiaoa, Wanjiang Wanga, Zongyi Shaoc, DianWei Qia,

Rong Wanga

aSchool of Civil Engineering, Xinjiang University, Urumqi, China bSchool of Civil Engineering, Bejing University of Technology, Beijing, China cBeijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, Beijing university of Civil Engineering and

Architecture, China

Abstract

A new solar energy-phase change storage-floor radiant heating system is proposed to provide a comfort indoor environment in winter. In this study the proposed new system is applied in an office building in Urumqi in China. The system consists mainly of three units: evacuate heat pipe solar collectors (as heat collection unit), vertical phase change thermal energy storage device (VPCTESD) and floor radiant heating system. The influence on thermal efficiency of solar collector, inlet and outlet water temperature, connect method during local heating period and thermal performance of this system was developed experimentally. The results reported in this paper adequately support the new system as an alternative to the traditional solar heating system, and it can increase 30% utilization of solar energy than traditional solar heating system. The research result provides a reference for optimization design of this kind of system.

© 2016 The Authors.Published byElsevierLtd. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of CCHVAC 2015

Keywords: solar energy application, phase change material thermal storage, radiant floor heating system, thermal performance;

1. Introduction

Foreseeable depletion of fossil fuels and CO2 emission has driven scientists to develop renewable energy systems. The heating energy in buildings is one of the leading energy consumers in China, and the building heating energy in northern China accounts for approximately 40% of total energy consumption in ChinaYi (2008). In order to improve energy conservation of buildings, solar energy is getting more attractive in building heating application in recent

*Ye Zhang. Tel.: 13999915050.

E-mail address: zhangye@126.com

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of CCHVAC 2015

doi:10.1016/j.proeng.2016.06.357

years as an abundant and clean source. Utilization of solar energy in building heating system can effectively reduce the building consumption and air pollution problem during winter.

Floor radiant heating systems are becoming increasingly popular due to the fact that they may provide a more comfortable indoor thermal environment than convective heating systems, and its relatively lower supply water requirement leads to the feasibility of solar energy (Olesen, 2002). Solar-powered floor radiation heating systems have been proven to be an appropriate choice in solar thermal utilization. Zhai et al. (Wang e Zhai, 2010) compared solar-powered floor heating system with the widely used air-source heat pump heating systems in Shanghai, the result shows great potential in energy conservation in winter, with respect to the whole heating period, the solar fraction was 56%. Alkhalaileh et al. (Alkhalaileh, 1999) investigated the potential of using solar pond floor heating system under Jordanian climate by simulation. It was found that the solar pond heating system could meet most of the winter season in Jordan with solar fractions in the range 80-100% for at least two months of the season. A theoretical and experimental study is made for under-floor heating system using solar collectors by Badran and Hamdan (Badran e A., 2004). Li (Li Yuanzhe, 2009) analyzed a solar floor radiant heating system in Beijing with air-source heat pump as auxiliary source. Martinez et al. (Martinez et al., 2005) analyzed the performance data of a residential solar radiant floor heating system in Murcia (Spain), and to compare the recorded data with the performance estimate provided by the f-chart method used for sizing the system. Kacan (Kacan e Ulgen, 2012) presented new approach on floor heating method by using copper finned-copper flow tubes for solar space heating. In all above solar floor radiant heating applications, the sparseness, discontinuity and instability of the solar energy are bottleneck of solar energy utilization. Therefore thermal storage tank is an important part in solar heating system, in order to save the space for water tank, phase change material (PCM) thermal storage technique is applied because of its high energy storage density and small temperature variation from storage to extraction, including PCM floor and PCM storage tank. In early 1978 Morrison and Khalik (Khalik, 1978) used simulation techniques to determine the performance of solar heating system utilizing phase change energy storage, the result showed that the storage volume required for phase change energy storage is considerably lower. Yuichi (Hamada e Fukai, 2005) studied the latent heat thermal energy storage tanks for space heating of building, the result shows that the proposed thermal

Nomenclature

to operative temperature in 'C;

ta air temperature in 'C;

tmrt the mean radiant temperature in °C;

Qsh accumulated storage heat, J;

Qrh accumulated released heat, J;

mpcm mass of DX-53, kg;

mw mass of water, kg;

tsst initial temperature of VPCTESD when storage heat, 'C;

tset final temperature of VPCTESD when storage heat, 'C;

trst initial temperature of VPCTESD when released heat, 'C;

tret final temperature of VPCTESD when released heat, 'C;

Cpcm specific heat of PCM in Eq.3,4, J/kg-'C;

Cw specific heat of water, J/kg-'C;

nhr efficiency of VPCTESD;

Qu useful heat gain the collector, J;

CHtF pecific is heat of HTF, J/kg-'C;

m mass flow rate of HTF through the collector, kg/s;

tso outlet temperature of collector, 'C;

tsi inlet temperature of collector, 'C;

Qsih the solar energy for indoor heating, J;

nsih the solar energy fraction for indoor heating;

nVpCTESp_the efficiency of VPCTESD for indoor heating._

storage material contribute to saving space and reducing the cost of the tanks. Huang (Huang et al., 2014) applied PCM floor in a solar water heating system which can greatly enhance the floor's energy storage capacity, and thus space for water tank is saved and heat loss at night is effectively avoided.

Based on the above review, it is found that few of these studies had considered the integrated solar floor heating system with the storage part. In this paper, a solar energy-phase change storage-floor radiant heating system was designed. The operation characteristics under typical weather condition of Urumqi were analyzed experimentally. Besides, the system performance was analyzed compared with an unheated reference room. The experimental investigation indicated typical operating characteristics of solar energy-phase change storage-floor radiant heating system. The design method and control strategy obtained through the experiment in this paper are instructional for the design of such a system.

2. System structure and operation mode

2.1. System structure

An experiment rig of solar energy-phase change-floor radiant heating system was established based on the former research theory. The principle of this combined system is using phase change thermal storage device store the surplus heat energy generated from solar collector when solar radiation intensity is high. The stored heat can be used during morning or afternoon when solar radiation is insufficient during winter daytime. The building heating consumption can be reduced through this approach. The whole system contains three main parts: Solar collector subsystem (SCS), phase change heat storage subsystem (PCHSS) and radiant floor heating subsystem (RFHS) (Figure 1). The collecting system, as the heat source of the entire system, mainly consisted of the heat pipe-vacuum tube solar collector, plate heat exchanger, pumps and pipes. To avoid the influence from the HTF on heating terminal, this experimental build used an indirect system with a plate heat exchanger. PCHSS consists of vertical phase change thermal storage device (VPCTESD), pumps, plate heat exchanger, water pipes, boiler, etc. Radiant floor heating system mainly consisted of radiation coil, manifold and valves, etc.

Figure1. Schematic of system

Figure2. Plan of experiment room

2.2. System operation mode

This system can operate in different modes in accordance with the changes of outdoor solar radiation. During daytime when the solar energy is sufficient, operate solar collector for heating and heat charge; when it's cloudy operate phase change storage device as a heat source for heating. Specific system modes of operation are shown in Table 1. In the morning when solar radiation is very low, let solar collect system working alone for preheat (Mode 1) until the outlet temperature can reach the requirement for supply heat to the room. At the same time the PCHSS can operate as the heat source for heating (Mode 4), and if it is snowy and cloudy, electric boiler can operate as heat source for heating (Mode 5). During daytime when the solar radiation is sufficient, solar collect system for heating (Mode 2) is used. In the noontime, the solar radiation is surplus; PCHSS for heat storage (Mode 3) is operated.

Tablel. Operation Modes of System

Mode Detail Solar Collector VPCTESD Operation

Mode 1 Solar collector for preheat Operate Stop Valve V1-V4 on, other valves off? pumplon

Mode 2 Solar collector for heating Operate Stop Valve V1-V6,V10,V13,V17,V18 off? pump1,2 on on? others

Mode 3 VPCTESD for heating Stop Operate Valve V9,V10,V13-V15,V17,V18 off? pump 2 on on? others

Mode 4 Solar collector for heating storage an Operate Operate Valve V1-V6,V10,V13,V14-V18 off? pump1,2on on? others

Mode 5 Electric boiler for heating Stop Operate Valve V7-V10,V13-V18 on, others off, pump 2 on

3. Experimental setup and test procedure

3.1 Facilities

The hybrid system is applied an office building in the fourth floor in Xinjiang University in Urumqi China. The plan graph of the experimental Rooml and Room2 is shown in Figure2. This two rooms have an external wall oriented west which is 340 mm cerement concrete, external window is double-glazed steel window. The other three sides of the internal walls are 100mm polystyrene with 0.8 mm steel on both sides. In present research, Room 1 is taken as heating room, Room 2 is non-heating room.

According to the former experimental result (Ye Zhang, 2012) 7 m2 evacuate heat pipe solar collectors (EHPSC) were used, placed tilted 50 °C on the roof of experiment building facing south to collect solar energy. The extremely outdoor temperature is -25 °C, to avoid frozen the heat transfer fluid (HTF) of the solar collector system is concentration of 70% propylene glycol 2, its freezing point temperature is -40°C, specific heat is 3160J/kg-°C and density is 1071kg/m3. A vertical phase change thermal energy storage device (VPCTESD) is adopted. The cross section size of VPCTESD is 600mm (length) *225mm( width), the diffusion length is greater than 180mm. The PCM material used in this experiment is DX-53 that is made by paraffin, graphite and high-density polyethylene. Table 2 shows the thermal characteristics of DX-53, Figure3 shows the specific heat of PCM DX-53 according to DSC DX-53 slabs are placed in VPCTESD parallel and vertically. The volume rate of EHPSC, VPCTESD and floor radiant system are respectively 0.2m3/h, 0.3m3/h and 0.18 m3/h.

Table2 Thermal characteristics of DX-53

Phase-change temperature °C Latent heat kJ/kg hermal conductivity W/(m-°C) Density Kg/ m3 Specific heat kJ/ (kg-"C)

50-55 144.6 39 900 (Figure8)

IS 25 35 45 55 (is Temperature $

Figure 3 The specific heat of PCM DX-53 according to DSC

3.2 Measurement system and data collection

Armoured thermometers were installed at the inlet and outlet of hot water loop to measure the supply temperature and return temperature with an error of ±0.5°C. A T-type thermocouple is also set inside the hot water tank to monitor the heat source temperature. According to(Regulations, 2002) surface temperature measurement points placed on floor, internal wall and window; vertical temperature from distance 0.1m, 1.1m, 1.7m to ground, these temperature will reflect the relationship between heated and unheated surface and indoor temperature profile. The inlet and outlet temperature measurement points of solar collect system were conducted following the Chinese standard GB/T4271-2007(China, 2007). Sensors for indoor temperature, ambient temperature, inlet outlet temperature of VPCTESD and solar collectors are PT-100 and T-TYPE thermocouple, with an error of ±0.1°C and ±0.5°C respectively, measurement data were collected automatically by Agilent 34970A data logger. Flow rate are measured both by electromagnetic flow meter and ultrasonic flow meter. Solar radiation on 50° tilt surface south orient is measured by TBQ-4 pyranometer with ±2% accuracy, and the data were automatically recorded at every 5s interval in data logger.

3.3 Performance indicators

Based on the previous analysis, it is found that a number of studies have been carried out to explore the solar collectors efficiency, the appropriate size of phase change thermal storage tank or the layout of the floor radiant heating system. Few of studies analyzed the overall thermal performance of the whole system. Therefore, in order to evaluate thermal performance of the proposed solar energy-phase change storage-floor radiant heating system in this study, followed indicators had been used.

3.3.1. Operative temperature

The operative temperature combines the effects from both indoor air temperature and the radiation from the building inner surfaces, it was used to assess indoor thermal environment(American Society of Heating e Engineers, 2004). For greater accuracy, the operative temperature was calculated using Eq.(1)

K = Aa + (1 - A) (1)

Where, to is operative temperature in °C; ta is air temperature in °C; the value of A can be found from the values below as a function of the relative air speed Vr. A is 0.5 when Vr is <0.2 m/s, is 0.6 when Vr 0.2 and 0.6 m/s, and 0.7 when Vr between 0.6 and 1.0 m/s; 'mrt is the mean radiant temperature in °C, which can be obtained by Eq. (2),

'mrt =Z(FA) (2)

In this study indoor relative air speed is<0.2 m/s during experiment process.

3.3.2 Performance of VPCTESD

As shown in our previous research (Chen et al., 2014), the accumulated sensible heat stored almost increase linearly as time increases in the charge process. The accumulated storage heat sh and accumulated released heat Qrh can be estimated by Eq. (3) and Eq. (4) respectively.

Q i, = m \'c (')dt + m \ " c dt

^sh pcm J' pcm ^ ' w J t w

Q u = m [ '" c (')d' + m [ '" c d'

^rh pcm J' pcm V ' w J ' w

where, Qsh-accumulated storage heat, J;

Qrh--accumulated released heat, J;

m -mass of DX-53, kg;

mw-mass of water, kg;

tsst-initial temperature of VPCTESD when storage heat, °C;

tset-final temperature of VPCTESD when storage heat, °C;

trst-initial temperature of VPCTESD when released heat, °C;

tret-final temperature of VPCTESD when released heat, °C;

cpcm-specific heat of PCM in Eq.3,4 which can be calculated by using differential scanning calorimetry

(DSC) curve of DX-53 PCM, detailed in Figure 3, J/kg-°C; cw-specific heat of water, J/kg-°C.

In this study the storage heat in VPCTESD is used for indoor heating in the morning when solar radiation is not high enough for provide indoor heating energy. In order to evaluate the contribution of VPCTESD to indoor heating, the efficiency of VPCTESD for indoor heating can be calculated in Eq. (5):

%r = ^ (5)

where, r/hr-efficiency of VPCTESD.

3.3.3 Utilization of solar energy

The useful heat gain the collector get is derived by applying the first law of thermodynamics to HTF. The result of this application, given below, assumes that changes in the kinetic and gravitational potential energies of the HTF as it enters and exits the collector are negligible, and the specific heat of HTF (propylene glycol ) remains constant at the average temperature of air in the collector, as Eq. (6).

Qu=™CHTF(tso-tSJ) (6)

where, Qu-useful heat gain the collector, J;

chtf-pecific is heat of HTF, J/kg-°C;

m-mass flow rate of HTF through the collector, kg/s;

tso-outlet temperature of collector, °C;

tsi-inlet temperature of collector, °C;

When solar radiation intensity is high during the noon, the useful heat gain from solar collector can be divided into two parts, one part for heating the room, the other for VPCTESD storage. The storage part is defined in equation (3), therefore the solar energy for indoor heating can be defined as Eq. (7),

Qh = Qu - Qsh (7)

where, Qsih-the solar energy for indoor heating, J.

In order to evaluate the useful heat gain for indoor heating, the solar energy fraction for indoor heating can be calculated in Eq.(8):

= Q (8)

¿¿ihr

where, -the solar energy fraction for indoor heating;

Qihr-heating energy requirement for indoor heating, J.

In order to evaluate the contribution of the VPCTESD to indoor heating, the efficiency of VPCTESD for indoor heating is defined as Eq.(9):

VPCTESD = Q (9)

where, -the efficiency of VPCTESD for indoor heating.

4. Experimental investigation

In this study, the experimental research was carried out from the winter of 2011 to 2014. Experimental results under typical weather condition of Urumqi were chosen to analyze the performance of the solar energy-phase

change storage-floor radiant heating system. The experimental results will be analyzed with four different cases, which is categorized with the typical variation of solar radiation as shown in table 3:

Table 3. Experiment cases

Case Solar Radiation (MJ/rf) Mode Operation Time

Mode1+Mode5 10:00-12:00

1 5-10

Mode2 12:30-18:00

Mode1+Mode5 10:00-11:30

Mode2 11:30-12:30

2 10-15

Mode4 12:30-16:00

Mode2 16:00-18:00

Mode1+Mode3 10:00-11:00

Mode2 11:00-12:00

3 15-20

Mode4 12:00-16:00

Mode2 16:00-18:00

4.1. Experimental result under different weather condition 4.1.1 Casel with daily solar radiation of 6MJ/m2

Figure 9 (a) shows the outdoor weather condition including temperature and solar radiation during operating hours in case1. The daily solar radiation was 6MJ/m2 and average outdoor temperature was -3°C. Figure 9 (b) shows the variations of inlet and outlet temperature of SCS, the solar collectors circulation for preheating from 10:00-12:30, before the outlet temperature of SCS rose up to 30°C. The electric boiler is operated as heat source for FHRS from 10:00-12:30. Fig 9 (c) shows the variations of indoor air temperatures in the Room1 floor heating room and the non-heating room Room2. It was observed that the average operative temperature of Rooml was nearly 21°C, which was higher than that of Room2 by 7°C. The same variation tendency of operative temperature in Rooml and Room2 can observed also in case2 Figure 10 (c) and in case 3 Figure 11 (c). This result shows that utilized solar energy-phase change storage-floor radiant heating system proposed in this study can achieve a comfort indoor environment.

Inlet and outlet temperature of FRHS and floor temperature of Room1 variations can also find in Fig 9 (c), in the morning the inlet and outlet temperature of FRHS increased in accord with the operation of electric boiler, the inlet and outlet temperature was slightly decrease when the system operation mode switched from mode 5 to mode 2, then varies in accord with the variation of solar radiation, decreased after 15:00 when solar radiation became to decrease. But the floor surface temperature didn't change obviously because of the FHRS has large thermal inertia.

4.1.2 Case2 with daily solar radiation of 13 MJ/m2

Figure 10 (a) shows the outdoor weather condition including temperature and solar radiation during operating hours in case2. The daily solar radiation was 13MJ/m and average outdoor temperature was -3.9°C. Figure 10 (b) shows the variations of inlet and outlet temperature of SCS, the solar collectors circulation for preheating from 10:00-12:00, and during this time the system is operated in mode 1 and mode 5. When the outlet temperature of SCS rose up to 30°C, the system operation switched to mode 2. When the outlet temperature of SCS rose up to 40°C, the operative temperature of Rooml is already 22°C, the system then operated in mode 4, the surplus thermal energy from solar radiation can be storage in VPCTESD. The hot water from the solar collector flowed into the top of the VPCTESD, and the water returning from the FHRS flowed to the bottom of VPCTESD. The variation of inlet and outlet temperature of VPCTESD is shows in Figure 10 (e); the total storage heat Qshwas 6.1MJ.

4.1.3 Case3 with daily solar radiation of 17.5 MJ/m2

Figure 11 (a) shows the outdoor weather condition including temperature and solar radiation during operating hours in case3. The daily solar radiation was 17.5 MJ/m2 and average outdoor temperature was -4.1°C. From 10:0012:00, the system is operated in mode 1 and mode 3. The VPCTESD is operated as heat source of FHRS when SCS is preheated, the hot water from top of VPCTESD flowed into FHRS, and the water returning from the FHRS flowed to the bottom of VPCTESD. The total released heat was 5.4 MJ. From 12:30 when the outlet temperature of SCS is greater than 40°C, the system is operated in mode 4, the VPCTSD storage the surplus heat energy from SCS; the storage is stopped when the outlet temperature of SCS started to decrease. The electric boiler didn't operate, which means the indoor heating requirement energy is 100% supplied by solar energy.

5. Discussion

Thermal performance of the system in each cases with various ambient weather conditions were analyzed. The performance indicators of SCS and PCHSS in each case are calculated in Table 4 in order to analysis their contribution to the indoor heating in detailed. It is obvious that the useful heat gain from solar collector is increased when daily solar radiation increased. The solar energy fraction for indoor heating is increased from case 1-3 (daily solar radiation is increased). In case2, the PCHSS can storage the surplus solar energy for indoor heating in second morning, the solar energy fraction for indoor heating is 41%.

Table 4. The contribution of the system for indoor heating

Case Average Outdoor Temperature Indoor Heating (°C) Requirement (MJ) Energy Qu (MJ) Qhr (MJ) Qhs (MJ) Hsih HvPCTESD

1 -3 25 8 - - 32% -

2 -4.2 29 18 - 6 41% -

3 -4.3 25 26.2 5.4 6.1 82% 22%

In case 3, the solar energy fraction for indoor heating is 82%, the efficiency of VPCTESD for indoor heating is 22%, the solar energy can satisfied the indoor heating requirement, by using the VPTSD the supplied heat energy is even higher than the indoor heating requirement, this can also explained that the indoor operative temperature is higher than designed requirement.

6. Conclusion

A solar energy-phase change storage-floor radiant heating system was designed and built in an experiment office room in Xinjiang University. The proposed system had been in operation in three winters from 2011-2014. The significant points of this experiment work can be emphasized as followed:

(1)The proposed system can improve indoor thermal comfort, in different outdoor weather condition; the average operative temperature of Room1 is 5-7 °C higher than the non-heating Room2.

(2)The VPCTESD can storage the surplus solar energy for second morning. With proposed control strategy, the application of VPCTESD can increase the utilization of solar energy by 30% than traditional solar heating system.

(3)When the solar radiation is greater than 15 MJ/m2, the proposed system is capable of satisfying indoor thermal environment by using 100% solar energy.

Acknowledgements

Funded by Beijing key lab of heating, gas supply, ventilating and air conditioning engineering: NR2016K05

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