Low temperature solar assisted (heating) system based on slurry PCM

Transcription

1 ANNEX 59 - High emperature Cooling & Low emperature Heating in Buildings Riqualificazione di edifici esistenti con elevati standard energetici: metodi e tecnologie ENEA - Via G. Romano 41, Roma - 12 giugno 2013 Low temperature solar assisted (heating) system based on slurry PCM Marco Perino Politecnico di orino Department of Energy (DENERG) marco.perino@polito.it Corso Duca degli Abruzzi 24, orino, Italy

2 INRODUCION - 1 he exploitation of solar energy for space heating purpose through traditional solar collector is often not profitable, since higher heating demands take place when low (or no) solar energy is available. (heating season: orino Italy, 2617 HDD)

3 INRODUCION - 2 he adoption of traditional thermal solar systems based on water and water solutions implies a relatively high working temperature of the heat carrier fluid ( 50 C 60 C being a typical range). his is required in order to: o provide with the HVAC terminals an enthalpy flux large enough with reasonable flow rates: H m c p out in o allow for sufficiently small energy storage systems: Q Vc p final start

4 hese constraints determines: INRODUCION - 3 o a reduction of the usability ( solar coverage ) of thermal solar systems (the solar fractions reduces with the increase of the temperature of the working fluid. Higher fluid temp. means lower collector efficiency). oan increase of the thermal losses in the distribution network of the energy losses of the thermal storage systems.

5 Where, m is the true (arithmetic) mean fluid temperature difference: a f,o f,i a m m 2 Δ here is a critical value of the incident solar radiation, G C, for which the efficiency becomes zero (losses equal the gains): INSANANEOUS EFFICIENCY - (EU MEHOD) 2 m 2 m m m i G Δ a G Δ a η G b G a ' F 0 2 m 2 m 1 C η Δ a Δ a G INRODUCION - 4

6 INRODUCION - 5 Lowering the critical value of the incident solar radiation, G C, allows to improve di UILIZABILIY of the solar radiation (i.e. its exploitability): I C 1h G C (I I C ) + d I 1h G d G C

7 INRODUCION - 6 Switching from the exploitation of the sensible heat to the exploitation of the latent heat allows to address these issues. Heat flux and thermal energy can be exchanged and stored at virtually constant temperature, still keeping reasonably small volume thermal storages and flow rates. Form:

8 Strategy and material selection - 1 One solution can be the use of traditional Phase Change Materials (organic/inorganic) storage systems coupled with a water (or other fluid) system. Nevertheless, this strategy implies heat exchanges with finite between the water/fluid flowing inside the solar collector loop and the PCM storage and between the water flowing in the HVAC terminals and the PCM storage. Each of these heat exchanges introduces irreversibility and energy/exergy losses. Water primary loop PCM Static tank Water secondary loop (radiant panels)

9 Strategy and material selection - 2 In order to eliminate these inefficiencies and to reduce the overall, a solar system based on Slurry PCM is proposed. Slurries stay always liquid (even if with a high viscosity) and can be pumped regardless from their state of aggregation. Slurry is created through a suspension of a micro-encapsulated PCM in a water Solution (different concentrations) he PCM can be used directly as a heat carrier fluid in the various loops of the thermal systems. wo system configurations can be adopted.

10 Configuration 1: wo open loops circuit System configuration selection - 1 o maximum theoretical thermal efficiency (no ), o More challenging from the technological point of view PCM primary loop 1 PCM dynamic tank Melted PCM ( 1) media( C) Δsolidificazione( C) 2 PCM secondary loop Solid PCM ( 2)

11 System configuration selection - 2 Configuration 2: One open loop circuit with Slurry PCM, one closed loop with water o Less efficient from the point of view of the thermal efficiency (one ), o Easier technological implementation. PCM primary loop PCM dynamic tank Water secondary loop

12 Research project : phases and activities State of the arte review, Analysis and selections of the suitable PCM, Characterization of the Slurry PCM and of its thermophysical properties (weakness: few studies and information are available on slurry PCM fluids. Durability and stability not well known). o Construction of a purposely designed laboratory test rig, o Laboratory characterization of the fluid (different PCM concentration dispersed in a water-glycol solution will be tested), Design and construction of a prototypal thermal solar collector (suitably designed and sized for working with the slurry PCM) Design and construction of a real scale prototypal thermal solar system, Monitoring of the solar system for at least one winter season.

13 Selection of the phase change material (n-eicosane)

14 Working fluid rheology - 1 he working fluid is not Newtonian, but pseduoplastic or dilatant?

15 Working fluid rheology - 2 A dilatant (also termed shear thickening fluid - SF) material is one in which viscosity increases with the rate of shear strain

16 Experimental test rig for measuring pressure drops Fluid features - 1

17 Fluid features - 2 he water phase of the working fluid is a mixture of water and ethylen glycol (40%). he microencapsulated PCM is a paraffin (n-eicosane), with a nominal melting temperature 37 C and a latent heat storage capacity of J/g in the melting range C); Different concentrations will be tested (spanning from 30 % to about 47.5 % of mpcm) For the numerical simulations the mpcm concentrations was 60% of the total mass of the heat carrier fluid, but this value is not practically usable (almost solid behavior) Concentration about 45 %

18 he fluid seems to be: HIXOROPIC, NON NEWONIAN, SIMILAR O A BINGHAM FLUID Fluid features - 3

19 Numerical models - 1 wo models have been developed (one in Excel spreadsheet, the other in the Simulink environment), Both water and spcm based systems were simulated and compared under the same boundary conditions, A parametric analysis was carried out for the typical meteorological year of urin, Italy ( N, E), by varying the tilt and azimuth angles. he solar collector efficiencies during January ( jan ), the heating season ( heating ), i.e. between mid-october and mid-april, and the entire year ( year ) were calculated.

20 Average collector efficiency year jan heating water spcm water spcm water spcm able 4.1 Efficiency of spcm and water solar collectors during the year ( year ), January ( jan ) and the heating period ( heating ), for different tilt angles (south oriented solar thermal collectors)

21 Design of the real scale prototypal thermal solar system - 1 Simulation of the energy demand profile (EnergyPlus) for a typical residential apartment (over the heating season and for a location in orino Italy, 2617 HDD, Heating season 15th Oct. o 15th April) INPU DAA FILE - Size - Materials and Construction - Simulation period - Weather file - Internal gains - People - Lights - Electric equipment - Radiant panels heating system SIMULAION OUPUS - Diffuse and beam solar radiation (W/m 2 ) on the solar collector surface - Zone Mean Air emperature ( C) - Plant Loop Heating Demand (W) - Zone Airflow

22 Design of the real scale prototypal thermal solar system - 2 Collector/Storage simulation Parametric Analysis Constant inputs Modifiable inputs - Mean hourly outdoor dry bulb temperature ( ) - Number of solar collectors - Mean hourly radiation received per m 2 of collector at operational inclination (G ) - Constant parameters and features of the solar collector - Accumulator sizing - Mean temperature of the mixture at the solar collector ( m ) Product operation ( C) 1( C) 2( C) media( C) C(kJ/kg) Microtek (MPCM 37D) Water

23 System performance Fluid Comparison (Energy kwh/y) Water PCM % of PCM improvement from water case Production Not satisfied demand Accumulated energy Energy loss Simulation results.

24 Conclusions and future ideas - 1 he system shows promising energy efficiency improvements, at least from the theoretical point of view, Improvements of the solar cover factor, Simple system from the installation point of view (no substantial changes with respect to traditional systems), Fluid features and properties not known. No experience and data available in literature, Material expensive (but probably due to scale effect), Impossible to work with spcm concentrations above 50 %, echnological issues to be solved as far as the slurry PCM is concerned (segregation, durability, ), he assessed (numerically) improvement of the efficiency of the solar collector alone is around 10 %.

25 Conclusions and future ideas - 2 Good tuning of the system concept with the idea of ABE (hermally Activated Building Elements - deep radiant systems in particular), specially configuration n 1. Possibility of exploiting the ABE as a larger thermal storage system, (with configuration n. 1). Future possibility of extending the idea also to the cooling (with different melting temperature of the PCM and by choosing proper heat sinks, e.g. Ground Heat Exchanger or foundations).