Annual Energy and Exergy Analysis of Single and Double Slope Passive Solar Stills

INTRODUCTION

The potable water is one of the basic needs for survival of human being. Due to fast increase in population in the world in general and India in particular, the potable water consumption has been increased tremendously. On the other hand available water in rivers, lakes and underground water has been polluted due to industrialization for up gradation of living standards for human being. Hence there is a need to develop self-sustained system to purify available brackish water into potable water for keeping healthy human being.

In order to meet the requirement of fresh water rigorous research have been carried out by various scientists on design, fabrication and development of solar stills for purification of brackish water. A Swedish engineer, Rudolph (1972) was the first who designed conventional solar still for supplying fresh water to a nitrate mining community in Chile, which was quite popular and was in operation for more than 40 years. Cooper (1969) discussed about the absorption of solar energy radiations in solar stills. Ahmad (1988) studied about single effect solar still with an internal condenser. Malik et al. (1982) reviewed the work on passive solar distillation system till 1982. Tiwari (2002) reviewed the work done on active and passive solar stills until 1992. Tanaka et al. (2000) developed a highly productive basin type-multiple effect coupled solar still. Tanaka et al. (2005) conducted indoor experiments for the vertical multiple effect diffusion- type solar still that consists of a number of vertical partitions in contact with saline-soaked wicks with narrow air gaps between partitions coupled with a heat pipe solar collector. Kurdish et al. (1998) have conducted performance testing and analysis of a double-glazed, air-blown solar still with thermal energy recycling. Faith (1996) and Delyannis (2003) have reviewed various designs of solar stills and studied the suitability of solar stills for providing potable water. Chaibi (2002) validated a simulation model for water desalination in a green house roof through laboratory experiments. Aybar (2006) developed mathematical model for an inclined solar water distillation system. Tripathi and Tiwari (2005) studied the effect of water depths on internal heat and mass transfer for active solar distillation system in winter climatic condition whereas Tiwari and Tiwari (2007a) studied the effect of water depths on heat and mass transfer in a passive solar still in summer climatic condition. Dayem (2006) demonstrated experimentally and numerically the performance of a simple solar distillation unit that is based on the multiple condensation-evaporation cycle. Tanaka and Nakatane (2006) carried out theoretical analysis of a basin type solar still with internal and external reflectors. Recently Tiwari and Tiwari (2007b) have reviewed the work carried out by various scientists till 2006. Dincer (2002) reported the linkages between energy and exergy, exergy and the environment, energy and sustainable development and energy policy making and exergy in detail. Hepbasli (2006) reviewed exergetic analysis and performance evaluation of a wide range of renewable energy resources, which includes solar thermal systems like solar collector, photovoltaic, hybrid (PV/T) solar collector, wind energy, geothermal energy, biomass and other renewable energy systems. Koca et al. (2007) has done the energy and exergy analysis of latent heat storage system with phase change material for a solar collector and they found that the average net energy and exergy efficiencies are 45 and 2.2%, respectively.

Mukherjee and Tiwari (1986) have done the economic analysis and cost analysis of three types of solar stills, viz. a single-slope Fiber-Reinforced Plastic (FRP) still, a double-slope FRP still and a double-slope concrete still and found that the cost of distilled water produced from conventional stills is minimum. Life cycle cost analysis of solar stills have been carried out by different researchers but none of them have carried out life cycle cost analysis of passive solar stills on the basis of annual energy and exergy analysis. In this paper an attempt has been made to analyses life cycle cost analysis and Energy Pay Back Time (EPBT) on the basis of annual energy and exergy performance of single and double slope passive solar stills.

EXPERIMENTAL SET-UP AND OBSERVATIONS

Cross sectional view of a schematic diagram of a single and a double slope passive solar still is shown in Fig. 1a and b whereas Fig. 2a and b is the photograph of the single and double slope passive solar still which is designed and fabricated for the experiment. The body of solar still is made up of a very low thermal conductivity material known as Glass Reinforced Plastic (GRP) of thickness 5 mm. The condensing cover of 4 mm thickness is made up of plane glass has been placed on the basin of solar still. The inclination of condensing cover for both single and double slope solar still is 15°. Rubber gaskets are used to make the solar still airtight. The bottom surface of the still was painted black to have high absorptivity of solar radiation. Solar stills are mounted on iron stand as shown in the photograph (Fig. 2a, b). The single slope solar still was kept facing due south and double slope solar still was placed in east-west direction to receive the maximum possible solar radiation. The output from the solar still is collected into a channel provided at lower side and is taken out through a pipe into a cylinder. The basin area for single and double slope solar still is 1 and 2 m², respectively and maximum water depth in case of single and double solar still can be 19 and 22 cm, respectively. It has been found that lower water depth of 1 cm gives highest annual yield thus the size of solar still to reduce the initial investment can be kept 5 cm (lower height) and 32 cm (higher side) for both single and double slope solar still (Table 1).

To predict the performance of single and double slope solar still experiments are conducted in the campus of Indian Institute of Technology, New Delhi in winter as well as summer months starting from October 2005 to September 2006 for three different water depths namely 0.01, 0.02 and 0.03 m. Experiments for each water depth are started at 7 am in the morning and lasted for 24 h. Following parameters are measured at an interval of 1 h.

Fig. 1a:	Cross sectional view of a single slope passive solar still

Fig. 1b:	Cross sectional view of a double slope passive solar still

Copper-constantan thermocouples are used with the help of digital temperature indicator having a least count of 0.1°C to record the water, vapor and condensing cover temperatures. The ambient air temperature is recorded with the help of a calibrated mercury thermometer having a least count of 1°C and the distillate output are measured by a measuring cylinder having a least count of 10 mL. The solar intensity was measured with the help of a calibrated solarimeter, having least count of 2 mW cm^-2.

The hourly experimental observation for a typical day (11.10.2005) of the year has been given in Table 2a. The average experimental observation for different depth and month of the year has been given in Table 2b. The average experimental data has been used to evaluate monthly output by multiplying it by number of clear days and the results has been given in Table 3a.

Fig. 2a:	Photograph of single slope solar still

Fig. 2b:	Photograph of double slope solar still

Table 1:	Dimensions of the experimental and proposed solar still

On the basis of predicted monthly performance of single and double slope solar still, the following studies will be carried out.

Table 2a:	Various measured temperature and yield for single and double slope solar still for a typical day 11, October 2005 for 0.01 m water depth

Table 2b:	Average ambient air temperature, solar intensity, water temperature and daily yield for single and double slope solar still for different water depth in various months of a year

MATHEMATICAL MODELING

Overall Instantaneous Thermal Efficiency of a Passive Solar Still
The overall instantaneous thermal efficiency of a passive solar still is given below:

(1a)

The daily yield can be obtained by adding hourly yield for a period of 24 h and can be obtained as

(1b)

The values of M_ew for different depth and months for single and double slope solar still are given in Table 2b.

The daily thermal efficiency is defined as:

(1c)

The daily thermal efficiency of single and double slope solar still is given in Table 5.

Exergy Efficiency of Solar Still
The general exergy balance for solar still can be written, Hepbalsi (2006), as:

(2)

Or,

(3)

Where, the exergy input to the solar still is radiation exergy and can be written as:

(4)

Where, A_s is area of solar still and I (t) is solar radiation on inclined glass surface of solar still and T_s is the Sun temperature in Kelvin.

(5)

Where:

(6)

The monthly exergy can be obtained by multiplying Eq. 5 by the number of clear days in Table 3a and results are given in Table 3b

The exergy of work rate for solar still is given by

(7)

The exergy destructed in solar still water is given by

(8)

Table 3a:	Details of monthly and annual yield obtained from single and double Slope solar still with different water depths

Table 3b:	Details of monthly and annual exergy output obtained from single and double slope solar still with different water depths

Where, M_W and C_W are mass and specific heat of solar still water, respectively.

The exergy efficiency of solar still is defined, Hepbalsi (2006), as follows:

(9)

The daily exergy efficiency is also given in Table 4b.

The exergy output of a solar still can be obtained as follows:

(10)

The daily exergy output will be sum of hourly exergy evaluated by Eq. 10 and the results are given in Table 4b

Life Cycle Cost Analysis
For a given initial investment, the uniform end of year annual cost of the passive solar distillation system can be written as follows (Tiwari, 2002)

(11)

Where, F_{CR, i,n'} and F_SR,i,n': represent the capital recovery factor and sinking fund factor, respectively and can be calculated by using the following formula:

(12)

Table 4a:	Monthly exergy of single and double slope solar still

Table 4b:	Daily exergy analysis of single and double slope solar still

And

(13)

The cost of distilled water per liter based on thermal energy can be calculated by dividing the annual cost of the system by annual yield of solar still. So,

(14)

Similarly, the cost of distilled water based on exergy can be calculated as follow.

(15)

Where, M_yen and M_yex is the annual yield of solar still based on thermal energy and exergy, respectively.

Life cycle cost analysis has been using Eq. 14 and 15 with the help of Table 3 and 6.

Capital cost, salvage value and maintenance cost of single and double slope solar still are given in the Table 6. These values are useful in determining cost of water as given in Table 7a and 7b on the basis of energy and exergy analysis.

Energy Pay Back Time (EPBT)

Energy Pay Back Time (EPBT) of solar still can be evaluated by using the following formula

(16)

Table 5:	Thermal efficiency of single and double slope solar still

Table 6:	Capital cost (Ps), Salvage value (Ss) and maintenance cost (Ms) of Experimental and proposed single and double slope solar still

Table 7a:	Cost of water from experimental and proposed single and double slope solar still on the basis of energy analysis

Table 7b:	Cost per liter from experimental and proposed single and double slope solar still on the basis of exergy analysis

Table 8a:	Energy analysis of single and double slope solar stills made for experimental setup

Table 8b:	Energy analysis of proposed single and double slope Solar stills

The annual yield for single and double slope solar still is given in Table 3a. The embodied energy of single and double slope solar still has been evaluated by multiplying mass of each component with their energy density as shown in Table 8a. Table 8b gives embodied energy of the proposed design as given in Table 1b.