“ SLOW FLOW “

D.S.E. group of COMPANIES

“ SLOW FLOW “

IN CENTRAL SOLAR SYSTEM

Abstract

Various single pass and multipass central solar heating systems are studied theoretically and examined from a system-life-cost aspect in order to arrive at a configuration yielding the highest benefit/cost ratio.

Based on such modeling and our extensive experience with various systems we submit that a Central Solar Heating System based on a modified Slow Flow system 1 Phase Change Material (P.C.M) in the storage circuit as designed and built by us meets these desired criteria.

Accumulated experience with dozen of systems based on our design and now operating over a prolonged period support our design and energy storage concept; they have 2 the desired high benefit/cost ratio.

1. Introduction

Recent studies of efficiency and cost effectiveness of thermosyphon solar energy water heating systems [1-3] compared the performance of Multi-Pass Systems (MPS) with that of Single Pass Systems (SPS) optional (and find them to be of 3 equal efficiency). The MPS is based on the premise, which preswaited for a long time that “every possible energy gain must be used”. In the MPS the water in the storage tank is caused to circulate through the solar heat exchanger several times during one day so as to utilize the solar energy to a maximum. This results in a system having relatively large pumps and pipes, a small temperature raise per cycle, minor thermal stratification in the water tanks and therefor small water temperature differences within the tanks.

By control the SPS or Slow Flow System. SFS is based on recirculation of the water through the solar field approximately once per day, raising the water outlet temperature to the desired usage temperature, in a single pass. The required storage tank volume in SPS is such as to suffice for daily usage. The pumps and the pipes used are smaller, slowing down the flow rate. Thermal stratification in the water tanks

is significant and the water outlet temperature is divide from the hottest water layer, nearest to the top of the tank.

Tabor showed [1] that under typical usage condition, and for climates as the type prevailing in Israel, the SPS yields a thermal efficiency approximately equal to that of a MPS, and that SPS has practical advantage that make it more cost effective. The work of tabor [1] was confirmed by the publication of J.Gordon and Y.Zarmi [2] who

extended the validity of Tabor’s conclusions to various climatic conditions. The advantages of the SPS using the slow flow principle as described by Tabor can found to apply also to central solar system supplying a 4 of consumers as well as to individual user systems.

In this paper we shell discuss design improvements in Central Solar Systems (which become popular in countries having many sunny days during the year) which offer short term and long term ecological benefits and economical benefits as they reduce the 5 and expenditures for required fossil fuels.

Central Solar Systems include –3- major units,

namely; A backup heating circuit,

A water storage circuit,

and A solar field circuit.

In the analysis, design and development of Central Solar Heating Systems described below we first considered and compared various features of the SPS and the MPS. In the solar heating system while we developed a phase change material is include in the storage circuit of a modified SPS while we have proven to be a cost effective system of high thermal efficiency. Several dozen of such central systems were installed by the chromagen Co’ in various locations and the accumulated experience validates the advantages of our design. Local authorities and governments have been subsidizing installation of Solar Heating System for

some time. We believe that performance improvements based on advances in design and technology shell help to substantially widen their usage in the near future.

2. Modeling and Design of Central Solar Water Heating System.

The function of a Central Water Heating System is to supply hot water to a client (hotel, apartment building, office building, etc) at a temperature and in quantities required. The hot water must always be available. The basic structure of such a system is shown schematically in fig 1.

Fig 1

Central Water Heating System

For most practical systems a hot water temperature in excess of 45 C deg is considered satisfactory. We shell next consider the:

2.1 Flow Rates and Attainable Efficiency in SPS. (The Slow Flow System).

In Slow Flow Systems one must control the water temperature so that it is at list at 45 C deg at the entrance level to tank #1 of fig 1.

Thermal loss in the solar heat exchanger and in the connecting pipes and the temperature difference need in order to maintain heat transfer in the heat exchanger regards a water temperature at the exit from the solar heat exchanger which is 5 C deg higher than the water storage temperature. A solar field water exit temperature of 50 C deg is, therefore, typically used.

The water outlet Tout from the solar field system is calculated on the basis of A.Whillier’s efficiency equation:

(1) = FR ( - ULT/It)

(2) T/It = [0.5 (Tin + Tout) –Ta]/It

And the heat balance equation [ ]:

(3) Qout/Qin = m Cp (Tout-Tin)/It

(4) Tout ={1/ (2 m Cp+FR UL)}[Tin (2 m Cp-FR UL)+2 Ta FR UL+2 It

FR ].

The outlet temperature from the solar field (Tout) can be controlled by varying the flow rate (m).

To investigate the various performance aspects resulting from use of the Slow Flow technique and the performance of a SPS, let as examine the various parameters in “Whiller’s” efficiancy equation:

(5) FR = F’ F” F’ is the collector efficiency factor.

(6) F’ = Co Ffe Ffe is the fin efficiency factor.

Co is a coupling parameter influenced by the thermal resistance between the pipe and the water flowing in the pipe, and by the thermal resistance mainly due to the joint between the pipe and the sunray collection surface fin.

In solar collectors using high-quality joints (such as collectors in which the fin is ultrasonically welded to the water conduits), the parameter Co depends mainly on the thermal resistance between the water and the pipe. The fin efficiency Fef is given by [ ] as:

(7) Ffe = tanh (m W/2)/(m W/2)

(8) m =

The flow factor F” is given by [ ] as:

(9) F” = Ao [1-exp (-1/Ao)]

(10) Ao = m Cp/(Ac Ul F’)

A plot of fin efficiency Fef on m W is shown in Fig 3 with t as a parameter.

Fig 3

fin efficiency Fef .

A plot of the flow factor F” as number of collectors is show in fig 4 (spec. flow value is parameter).

Fig 4

F” flow factor.

For climatic conditions in the northern Mediterranean basin a chromagen CR-120 collector typically provides solar field temperatures of 50 C Deg. As the specific flow rates in the range of 8 l/m^2h to 20 l/m^2h. Under climatic condition of central and northern Europe solar field temperature of 50 C Deg. Are attained with such collectors at specific flow rates in the range of 5 l/m^2h to 10 l/m^2h. [(6,2),(6,3)].

2.2 Heat Transfer Efficiency As a Function of Water Flow Through the

Collectors.

The rate of heat transfer from the collection surface to the flowing in the conduit was seem to be determined by Fef and F”, were:

Fef – the fin efficiency (Fig 3) depends on geometry and on k, the thermal conductivity of the collection surface material.

F” – the flow factor (Fig 4) depends on the water conduit flow rate. In order to operate in the slow flow regime and to obtain required field outlet temperatures of 50 C deg. The specific flow rate common in MPS should be typically reduced by a factor of 3 to 4. Such a reduction significantly reduce the flow factor F” value and causes an appreciable decrease in the efficiency of individual solar collectors.
The achievement of high overall field efficiencies at low specific flow rates requires maintenance of a high flow factor in every collector in the solar field.

This can be attained by a series connection of the collectors as shone in Fig 5.

Fig 5

Series (Serpentine) connection of collectors

A parallel connection as shone in fig 6 is also possible.

Fig 6

A parallel connection of collectors

For a parallel connection as shone in Fig 6 the number of joint is greater, the piping is longer, the hydraulic resistance is higher, and the heat losses from the connecting pipes are greater.

2.1 Solar Field Efficiency for a Series (Serpentine – Connected) Collector System.

Use of a serpentine connection solves the efficiency problem due to slow flow, but leads to a decrease in efficiency, due to flow in a direction opposite to the thermosyphon direction.

Efficiency of a solar collector, with reverse flow was examined in an experiment performed in the solar laboratory of Lordan Ltd. In Israel, by D. Lorence. [ ] .

The observed results can be expressed by the following flow factor equation which contains a correction factor [ ].

( 11 ) Rff = Ao’[1-exp(-1/Ao’)]

( 12 ) Ao’ = m Cp /[Ac Ul (F’/F”’)]

Eq. (11) and (12):

Rff is the Reverse flow factor.

F”’ is the Reverse collector efficiency and the other symbols are as previously describe.

With serpentine connected collectors flow in the thermosyphone direction takes place in about half of the collectors; flow in the opposite direction takes place in the remaining collectors.

Consider a model composed of Series collectors connected in serpentine as shown

In Fig. 7

The overall efficiency for such a model is given by:

(11) F_R={ [1-(1-K)^N]/(NK) (FR)₁

Where (F_R)₁ and (F_R U_L)₁are the values for the original test of the collector that was made according to the standard.

(12) F_R U_L={ [1-(1-K)^N]/(NK) (F_R U_L)₁

Were

(13) K = (F_R U_L)₁/ (m N Cp)

And N is the number of collectors connected in serial.

The efficiency of a row of collectors (the efficiency of a row is equals the efficiency of the solar field when the rows are thermally balanced) is given by:

(14) Field = F_R (-U_L T/It)

Fig 8 represents the solar field efficiency (the efficiency of a row of collectors as a function of specific flow rate and of the number of the collectors connected in serpentine.

N = the number of collectors in a row.

2.2 Schemes for Efficient Improvements in MPS.

A commonly used collector connection for a MPS is shown in Fig 9.

*[[The efficiency of the solar field using a connection as shone in Fig 9 is a parameter of the flow rate through the collector, the heat loss and the reverse thermosyphon flow in the connecting piping]].

The efficiency of the solar field, improved if the outlet temperature from each row of collectors is identical.

Riner, Croy and Felix Peuser [4] point out a significant improvement in the hydraulic balance of the solar field and an increase in the solar efficiency if one employs a serpentine connection, instead of the common type of connection as shone in Fig 9.

The pressure drop along a row of serpentine-connected collectors is high and permits good hydraulic- balance even at low flow rats in systems employing small-bore piping and other small bore accessories.

The use of flow rates which are three to four times lower, by using of serpentine connection significantly reduces the cost of pump, the cost of the electricity consumed in operating it, and the piping as a result of reduction diameter.

3. Flow Rate Control Strategies and Efficiency Optimization in MPS and SPS.

The flow rate control strategy in MPS is such as to take full advantage of every possible energy gain; use of a differential thermostat enable implementation of this strategy.

The control strategy in SPS dictates activation of the pump in the solar field circuit, when the water temperature at the outlet from the solar field, is equal to, or above 50 C Deg and activation of the storage pumps when the temperature at the input to the storage tank is above 45 C Deg.

A control system, which regulates the flow rate and ensures that the outlet temperature is lower then a set control value is suitable for implementation of this control strategy.

In MPS, the water is cased to circulate through the storage system several times per day and the water temperature increases by a few degrees for each cycle.

Rapid circulation of water in the system causes mixing in the tanks and results in a small temperature difference between the various storage tanks and between water layers with no stratification its need to wait until water is heated to user temperature.

In a SPS the water flow rate is controlled by monitoring the water temperature at the outlet from the solar field. The flow rate in the solar field and in the storage circuit is controlled so as to keep A desired temperature of the water leaving the solar heat exchanger and entering the storage circuit.

This control strategy causes stratification of temperature in the storage tank. Wuestling [7] presented simulation study results indicating a energy utilization of 66% as a result of slow flow and a highly stratified layer of a solar system operating in the sloe flow regime in comparison to a multi pass system, where the solar energy utilization reaches only 48% as a result of water mixing.

In order to reduce the operating time (and reduced consumption) of the Backup Heating System (BHS) the BHS should be installed as a water flow heater near the outlet of the storage circuit. Thus the heated water enters the circulation circuit at a usable temperature and returns to the storage circuit at a lower temperature. This results in a saving of energy stemming from heat loss reduction due to lower water temperature of the storage circuit and of the connecting piping.

Low storage temperatures also reduce build up of calcium deposits in the system.

4. Improved Energy Storage System.

A major problem encountered in the design and installation of a central solar system in existing buildings is a shortage of storage capacity. Enlargement of storage capacity is costly, problematic, and often difficult to carry out. Unique method for storing energy in central solar system was developed.

Tank containing (phase change material) PCM within the storage circuit.

A schematic diagram of a Central Solar Heating System employing PCMs is shown in Fig 10

Fig 10

This technology and method enable one to significantly increase the energy stored at small additional cost without resorting to the construction of additional utility rooms. Employment of this technology stabilizes the water temperature in the storage circuit at 53-55 C Deg depending on the PCM used.

Use of PCM to increase the Effective Storage Capacity

Conclusions

Central Solar Water heating systems based on MPS and SPS were considered and SPS was examined in great detail. In comparing the two system types one found that as far as: solar field efficiency, storage circuit efficiency and cost of connecting pipes accessories and pumps the SPS is the preferable system. As far as the cost of the control system and the heat exchanger the MPS is at an advantage.

If an overall system cost / profit value is taken as a yardstick for system efficiency the SPS method is superior in pachculas if the PCM technology is used in the storage circuit.

Conventional SPS were originally developed for individual systems. Today about 50,000 individual units are in use, most in Israel and the others in Europe, Asia, Africa, and America.

The SPS (or slow flow systems) design and construction care subsequently extended to Central Systems. Over 30 central systems were and operate successfully in Spain, Chili, and Greece and provide us with extensive confirmation of the validity of my design and of the consideration persuated in the paper. It also reason as a stimulus for further development of SPS central solar systems which possible an Accumulated experience with dozens of systems based on my design and now operating over a prolonged period supports our design and energy storage circuit concept; they have provide benefit / cost ratio. Even higher cost/benefit ratio. The experience which is being accumulated with the installed systems provides feedback which shell lead to further advances in system design and to improvements as related to installation and maintenance of central solar systems.

Reference:

1) H.Tabor., (1969) A note on the Thermosyphon hot water heater.

Cooperation Mediterraneenne Pour L’Energie,

Bulletin No 17, pp. 33- 41.

2) J.M. Gordon and Y. Zarmi., (1981) Technical Note, Thermosyphon System: Single VS multi-pass.

Solar Energy Vol. 27 No 5, pp. 441-442.

3) Y.F. Wang, Z.L. Li and X.L. sun (1982).

A “ONCE-THROUGH” SOLAR WATER HEATING SYSTEM.

Solar energy Vol. 29 No 6, pp. 541-547.

4) Reiner Croy and Felix A. Peuser., (1994) Experience With Solar Systems For Heating Swimming Pools In GERMANY.

Solar energy Vol. 53 No 1, pp. 47-52.

5) A. Arbel and M. Sokolov., (1994) Improving Load Matching Characteristics Of a Thermosyphonic Solar System By Thermostatically Controlled Circulation.

Solar energy Vol. 52 No 4, pp. 347-358.

6) 1993 ISES Solar World Congress.

6.1 – Peter Fagerlund Carlsson., Heat Storage For Large Low Flow Solar Heating Systems.

6.2 – M. Mack and C. Funfgeld., Low Flow System Design Consideration.

6.3 – C. Funfgeld, M. Mack., Side By Side Comparison Of a Low Flow and an

Advance Conventional Solar Domestic Hot Water System.

6.4 – Karsen Duer and Svend Svendsen., Conversion Of Indoor Measurements To Outdoor Long-term

Performances For Low Flow Solar Collectors.

6.5) – Simon Furbo., Optimum Design Of Small DHW Low Flow Solar Heating Systems.

6.5) John A. Duffie., William A. Beckman., (1991) Solar Engineering Thermal Processes. Jhon Wiley & Sons Inc., Second Edition pp. 498.

DESIGN OF A SOLAR CENTRAL SYSTEM AT THE ROBINSON LYTTOS HOTEL IN CRETA GREECE

Method, Design Equipment and Cost.

The solar system will be based on the “Slow Flow” Concept, which enables the operation at higher efficiencies, compared to standard one. The solar system will be designed by using different computer programs.

1.0 Design details

1.1 Program 1.

This program performs solar calculation, which are based on average climatic data at a specific location.

The program analyses, the data builds a model that describes a characteristics day, using this program for the calculation of a module day.

a. Number of collectors’ need?

· Number of collectors in a raw.

· Pressure drop on collector’s line.

b. Total volume of the storage tanks.

c. Power of the heat exchangers?

d. Pump flow rate and pressure drop?

1.2 Program 2.

Drawing software.

a. Solar field construction:

The collectors will be drawn on the roof taking care on:

· Obstacle.

· Shadow line.

· Number of collectors in a raw.

· Cold water line.

· Hot water line.

· Sensor location in the solar field.

· A characteristic collector line.

· Expansion tank (open/close).

b. Machine room construction:

The machine room will be constructed, To enable the optimal operation with the Solar system. The new design will include:

· Number of storage tanks.

· Total volume of storage tanks.

· Configuration of the connecting pipes between the storage tank, solar heat exchanger, backup system, backup heat exchanger and mixing pump.

1.3 Program 3.

The hydraulic software will be used for the calculation of the pipes diameters at the solar field and in the machine room.

Attention will be drawn to keep engineering standards: maximum allowed velocity, and further engineering needs: like equal flow rate in any collector raw. The pump curves will be included into the software and as a result the correct pump to suite the flow and pressure which will be found.

1.4 Control and Monitoring System (C.M.S).

The Control System operates the solar when energy is taken from, and operates the backup system when needed.

The monitoring system unit will collect and keep (in real time) the information when energy produced by the solar system and or by the backup system.

The control will be build as a module unit called “Brain”. The “Brain” will be constructed as a box. The box assembly unit will include all the sub systems that need to operate the solar system.

Sub systems:

· Energy systems.

· Pumping systems.

· Control instruments.

· Control systems.

· Electrical systems.

The control module unit will build to operate and check in Israel, it will be transported to the client, connect to the solar field, the storage system, and to the electricity. Fine-tuning will be made when operating the whole system, throw the Internet.

2.0 Cost brake down.

2.3 Part list of the “Brain”:

We can’t give accurate price, due to lack in information.