Efficiency of solar energy use for residential heating and cooling

Efficiency of solar energy use for residential heating and cooling

03MSM2/8I/OM)S19-09502.00/0 Copyright 0 1981 Pergamon Press Ltd. Energy Vol 6. pp 519-527. 19x1 Punted m Great Briraln. All wghts reserved EFFICIENC...

498KB Sizes 0 Downloads 20 Views

03MSM2/8I/OM)S19-09502.00/0 Copyright 0 1981 Pergamon Press Ltd.

Energy Vol 6. pp 519-527. 19x1 Punted m Great Briraln. All wghts reserved

EFFICIENCY OF SOLAR ENERGY USE FOR RESIDENTIAL HEATING AND COOLING T. MUNEER and M. HAWAS Mechanical Engineering Department, Faculty of Engineering, P.O. Benghazi, Libya (R&d

BOX

9476. University of Garyounis,

22 September 1980)

Abstract-The possibility of using solar energy collected on flat plate collectors situated on roofs for residential space heating/cooling and domestic water heating is considered. The study is carried out on a typical house situated in various locations in Libya. Two types of constructions involving heavy and light insulation, three roof tilts, and three values of system efficiency are considered. The study shows that the demand in a great part of the country can be provided from solar energy by a medium efficiency system, even with light insulation and a horizontal roof. Only in a few locations should the roof be tilted at an angle of lo”. For a low-efficiency system, insulation is necessary; for a high-efficiency-system, it was found that there is no need for either heavy insulation or tilting of the roof. NOTATION A area, m* b tilt angle, degree DD degree-days per month, “C K constant in Eq. (3). W/m2) L monthly total load, MJ L l?i” monthly appliances cooling load, MJ

monthly body cooling load, MJ monthly space cooling load, MJ monthly space heating load, MJ LH L HW monthly domestic water heating load, MJ L 4 monthly heating (or cooling) load due to infiltration, MJ L,, monthly transmission load, MJ number of days per month Qii rate of heat loss (or gains) due to infiltration, kJ/hr Q,r rate of heat transmission, W I, ambient temperature, “C inside design temperature, “C : overall heat transfer coefficient, W/m*-“K V volume of outside air entering the building, m’/hr Lb

Lc

INTRODUCTION

Space and domestic water heating for residential houses is the most common application of solar energy. Space cooling has also proved to be economically feasible in many locations. Flat plate collectors are usually employed for these applications. The roofs of residential houses are areas ideally suitable for collecting solar energy.’ The main objective of the present work is to investigate the feasibility of using solar energy for residential space heating/cooling and domestic water heating. Three factors must be considered: (a) the amount of energy required, (b) the amount of solar energy available, and (c) the efficiency of conversion of solar energy to the required demand. The demand depends on the structure of the building and on weather conditions. The amount of solar energy available depends on the location and the collecting area. The overall system efficiency includes the efficiency of collection, storage and conversion to the required form of energy. These factors are analysed and applied to various locations in the country. ENERGY DEMAND

Usually space heating is required when the ambient temperature is less than 18.3”C (65°F) and cooling is required when the ambient temperature is higher than 25.6”C (78°F). Domestic water heating is required throughout the year. Table 1 shows the mean monthly temperature and the degree-days per month for the locations considered. A single-family, double-story house is considered as a model for a typical residence that meets the traditional dwelling demands in Libya. The construction details of the house are 519

.Tripoli

1 Benghazi

Ghadames

1 Jalo Sebha

I

s Kufra

Table 1. Weather data for various locations in Libya; t. = mean monthly temperature (“C); DD = degree days per month (“C); the indoor design temperature is 18.3”Cfor heating and 25.6”Cfor cooling; an asterisk (*) denotes cooline.

521

Solar energy use for residential heating and cooling

shown in the Appendix. Two types of constructions are considered: Type I with heavy insulation and Type II with light insulation. Table 2 lists a summary of design data for each type. The heating load consists of transmission and infiltration loads. The cooling load consists of transmission, infiltration, body, and appliance loads. The calculation of the loads is carried out according to the procedure of Ref. 2. The rate of heat transmission Qtr is

The monthly transmission load is then L = 22.67 DD MJ (Type I), ,r 72.00 DD MJ (Type II).

(1)

The rate of heat loss due to infiltration (Qin,) is given by Qinf= 1.208 V (ti - t,) kJ/hr, where V is the volume of outside air entering the building (m’lhr). The monthly infiltration heating load is then L,, = 6.7 DD MJ.

(2)

The rate of heat gain associated with infiltration (@in,)is given by Qi,, = K x A, W) where A is the gross external wall area (m’) and K is a constant depending on the ambient temperature (W/m2). The monthly infiltration cooling load is then Li,f = 27.65 K n

MJ,

(3)

where n is the number of days per month. The body cooling load is taken as 225 Btuh per person. For a family of six persons, the monthly body cooling load Lb is Lb = 34.182 x n

MJ.

(4)

Table 2. Summary of house-design data; Type I refers to heavy insulation; Type II refers to light insulation. For construction details, see the Appendix. Item

Type I

Roof arcia (horizontal)

(~2)

1; 6

Net area 'f extrrna1 walls (m2) External glass area (doors & windows:

Type II

256 (m2)

t4

Overall heat transfer crefficien: uwali(LIl~2 OK)

0.432

2.66

Jglass(W/m2 OK)

2.38

2.38

(‘JA)wall

(W/K)

Iii

681

(UA)glass

(W/OK)

152

152

(WA)

263

a33

Overall heat transfer coefficient

t('JA)

522

T. MUNEER and M. HAWAS

The cooling load caused by appliances is taken as 1200Btuh. Considering six hours as an average working period per day, L,, = 7.596 x

n

MJ.

(5)

The latent cooling load is taken as 25% of the sensible cooling load. The domestic water heating load is considered to be 11,000Btu per day per person.’ For a family of 6 persons, the monthly load is L HW= 69.63 x n

MJ.

(6)

Table 3 shows the various loads in one city for each month. Table 4 shows the annual loads for all locations considered. AVAILABLE

SOLAR ENERGY

Monthly average values of solar radiation incident on tilted surfaces are available at the locations considered in this work.4 Table 5 shows the annual insolation values on surfaces facing south and tilted at angles of 0, 10, and 30” from the horizontal (for Kufra, a 25” tilt angle is considered). The tilt angle of 30” (or 25”) is nearly equal to the latitude. The roof of the residence is considered in this work to be the collection area. To increase the incident solar energy, the roof may be tilted. Three tilt angles are considered, namely 0, 10, and 30” (25” for Kufra). The horizontal roof area is 128 m*; for other angles, the area is obtained from simple geometric relations. All roofs point south (see Appendix). OVERALL

SYSTEM EFFICIENCY

Only a fraction of the incident solar energy can be utilized. This result follows for two reasons; firstly, not all of the incident energy can be collected and this limits the collector efficiency and, secondly, only a fraction of the collected energy can be utilized and thus limits the utilization rate. The collector efficiency depends on various factors, including the collector temperature, optical characteristics of the covers and absorbing plate, and heat-transfer properties of absorber and fluid. The collector temperature, in turn, depends on the application; for space cooling, it is higher than for space and domestic water heating (resulting in lower collector efficiency for cooling applications). For heating purposes, the collector efficiency may be as high as 70% but it can be as low as 30% and it is reasonable to assume an average value of. 40%~’ For cooling purposes, the collector efficiency is lower. In some existing applications for both heating and cooling, the collector efficiencies range between 22 and 35%.5 Therefore, a value of 30% for the collector efficiency seems reasonable for this work. A significant amount of the collected energy will be wasted because the supply and demand are out of phase. The utilization rate is defined as the ratio of the utilized energy to the collected energy. The utilization rate depends on the system and the energy-use pattern. It is very difficult to estimate a reasonable average for the utilization rate. Values of 4&80% have been reported.’ A value of 50% was suggested as a rough guide.5 It seems reasonable to consider utilization rates of 40&l%. The overall system efficiency is the ratio of the utilized solar energy to the incident solar energy, i.e. it is the product of collector efficiency and utilization rate. In view of the preceding discussion, three values for the overall system efficiency (10, 15, and 20%) are considered to correspond to low, medium and high efficiency systems. RESULTS

AND DISCUSSION

Table 6 shows the annual energy demand for space heating, space cooling and domestic water heating for the two types of building construction. Type I has heavy insulation and Type II has light insulation. The table shows also the net available solar energy (incident energy x overall system efficiency) for the three roof tilt angles and for the three values of overall efficiency. Figures 1 and 2 show the energy damand and the net energy available for an overall system efficiency of 15%.

Location

I

._

March

.

1

February

January

Month

"inf

arcs_

.Ltr

1 18627 ^ ^ I

_____ I

14285

I

1 1

.LH

I

7204

!'H I

I

I

--

.LC

29562

4851 21412

.LC

I

'L

Type

__

82747

'LH

'Type II 'LC

I

'L

19002

13598 6566

21677

L

1 154164

____

--

--

--

LC

II

---_-

19518 _,,_.

LH

10656 11648 4032 4407

17856 -,-,-

Ltr

93888 57374 71371

3804

6297

944z , ,

L

1 8445 1 15408~16843~-----

'Ltr

35916

1 6286 1 ----

1 3355 1 4347 1 -1 1270 1 1645 1 __-

5622

.Ltr

Type

Table 4. Annual demand (MJ)

992 375

1662

-Type I

17.322

15475

17343

1

12251

13104

1

b = 30'

11469

b = lo0

10158

b = 0'

,Linf

Table 3. Available energy and demand for Ghadames (MJ). The overall system efficiency is 20%.

.Net available Solar,energy

T. MUNEER and M. HAWAS

524

Table 5. Annual solar radiation (MJ/m*)

F?mghazi

320 06’

6547

6986

7104

Ghadms

30" 06’

6095

7252

7503

Jal0

2g" 10'

6990

7325

7525

Sebha

270 02’

7214

7550

7694

K&i-a

24' 14'

7520

7834

7965 (at b=25')

Table 6. Annual net available energy and demand (10sJ). An asterisk (*) indicates that the net available energy is less than the demand for Type II construction. ,Lcad .Location

611 Tripoli

1040 Benghazi

Net available solar energy

TYPO I Type1 O0

814*

1220

1627

10C

870'

1306

1741

30°

1025*

1537

2050

480 780 827 1542 670

Jalo

1137 772 1390

Kufra

729 1278

Comparisons of the energy demand and net available solar energy allow determination of the answer to the principal question, i.e. whether or not the net available energy is sufficient to meet the requirements. For heavy insulation (Type !), the available energy satisfies the demand even for the low-efficiency system and a horizontal roof. For light insulation (Type II), the low-efficiency system is not adequate to satisfy the demand in any location, even at the highest tilt angle, except for Benghazi where the demand is relatively small. The medium efficiency system can provide the demand in most of the cities, without the need to tilt the roof. The demand in any location can be provided if the roof is tilted at an angle of IO”. The high-efficiency system provides the demand in all locations without tilting of the roof. We have not studied economic feasibility. Instead, we have attempted to provide an answer to the possibility of providing the demand by solar energy means using systems which are currently available. The study shows that this is possible. An economic study is needed to determine the optimum combinations of system efficiency, insulation level and roof tilt angle.

Solar energy use for residential heating and cooling

rI

,'

_--

, ;

*\

TRIPOLI ‘.-_-o.._

1220/1306/1537

-\ '.__

611

*/' /'

,_,-

\ I \

__‘-----.,_ 3

~/BENGHAZI

‘L---.,m

__

1257/1343/1577 480

\ --__-~.

1’

‘_I

I





e-

0 GHADAMES / 1324/1414/'1666 / I

‘_

‘\

‘.

..__F

-p-

827

I'

JALO 0 1342/1428/1671 670

SEBHA 0 1385/1472/1708 772

KUFRA

\ ‘.__-. '\

iL44/152E/t685 \

.

Cl

729

Fig. 1. Available solar energy and demand (Type I). The overall system efficiency is 15%.The solar energy at O!IO/?O” tilt angles (10sJjyr) is shown divided by the demand.

T. MUNEERand M. HAWAS

526

I-.

: ‘-‘___Q. : _ ,-‘1220/1306/1537 _,

TRIPOLI --_ .\._

1040

/’

:

I

__I

,*’

‘_~~----,____

l257/1343/1577 780

,/’

GHADAMEZ

f

~BENGHAZI

1324/1414/1666 1542

8’ :

JALO 0

\ ‘, , I , ,

SEBHA 0

%. I ;

1342/1428/1671 1137

I I , I

1385/1472/1708 1390

I I I I I 1 I I I I I I I

‘\ -\ -\

\

\

.__-.

‘\

KUFRA 0

1444/1528/1685 1278 ‘\

‘\ \\

‘_

---___

,_.

. . _.-__

-\

\ ‘=_._

‘.

I I I 1 I 1

‘\ ‘=__

-.

‘.

‘,

. .

‘.

‘.

.____J I I I I

. . ‘-._ .. .

_.

. .

. .

‘-1

Fig. 2. Available solar energy and demand (Type II). The overall system efficiency is 15%.The solar energy at O/10/30”tilt angles ([email protected] J/yr) is shown divided by the demand.

527

Solar energy use for residential heating and cooling REFERENCES I. R. C. Neviile, Solar Energy 19,539 (1977).

2. ASHRAE Handbook of Fundamentals, New York (1977). 3. J. F. Kreider and F. Kreith, Solar Heating and Cooling. McGraw-Hill, New York (1977). 4. M. Hawas and T. Muneer, Energy Conv. Management 20,213 (1980). 5. S. V. Szokoiay, Solar Energy and Building, 2nd Edn. Wiley, New York (1978).

APPENDIX:

HOUSE DESIGN

Figure Al shows the layout of the house. The principal dimensions are as follows: net floor area (ground and first floor) = 210 m2, gross floor area = 260 m2, roof area (horizontal) = I28 m’, net area of external wails = 256 m2,area of doors and windows (all glass) = 64 m2. Two types of insulation for the external walls are considered: Type I (heavy insulation)

has IOcm concrete brick, lOcm mineral wool, and IOcm concrete brick with 1.5cm plaster on each side. Type II (light insulation) has a 20cm hollow concrete block with 1.5cm plaster on each side. The doors and windows ail have double glazing (fin. air space).

I Plan of

the

ground

g’

Sun&

Entrance

floor

courtyard

kitchen

Kitchen courtyard

La

ra

Ld Section

PIan

a-a

Fig. AI.

of the

first

floor