Placement of Sensors

Proper placement of the collector temperature sensor is important for good system operation. The sensor must have a good thermal contact with the collector plate or piping. Collector sensors may be located on the collector plate, on a pipe near the collector, or in the collector outlet pipe. The best of all is on the collector plate, but this is not the easiest, because dismantling and modification on one collector of the array is required, which would need to be done on site. The easiest and perhaps the best point for the location of the sensor is on the pipe leaving the collector. Usually a T piece is used and the sensor is placed in a deep well with a few drops of oil, which ensures good contact, as shown in Figure 5.30a, or on the side of the T piece, as shown in Figure 5.30b.

The storage tank sensor should be located near the bottom of the storage tank, at about one third of its height. If the system uses an internal heat exchanger, the sensor is located above the heat exchanger. Ideally, this sensor should identify if there is still water in the tank, which can be heated by solar energy. Therefore, the location indicated is considered a good compromise because a lower location would give a false reading even with the slightest demand, which will be replaced by make-up (cold) water, whereas a higher location would leave a lot of water at a low temperature, even if solar energy is available.

A freeze protection sensor, if used, should be located in such a position so as to detect the coldest liquid temperature. Two suitable locations are the back of the absorber plate and the entry pipe to the collector from the supply manifold. For the reasons indicated previously, the latter is preferred. The over-temperature sensor can be located either at the top part of the storage tank or on the collector exit pipe. For the latter, the sensor is located in a similar location and manner as the collector temperature sensor.

5.6 HoT wATER DEMAND

The most important parameter that needs to be considered in the design of a water heating system is the hot water demand over a certain period of time (hourly, daily, or monthly). The energy demand, D, required for the generation of sanitary hot water can be obtained if the volumetric consumption, V, is

Table 5.6 Typical Residential Usage of Hot Water per Task

Use

Flow (L)

Food preparation

10-

-20

Manual dish washing

12-

-18

Shower

10-

-20

Bath

50-

-70

Face and hand washing

5-

-15

known for the required time period. Also required are the temperatures of the cold water supplied by public mains, Tm, and the water distribution, Tw. Then,

If the two temperatures in Eq. (5.65) are known for a particular application, the only parameter on which the energy demand depends is the hot water volumetric consumption. This can be estimated according to the period of time investigated. For example, for the monthly water demand, the following equation can be used:

V Ndays Npersons Vperson (5.66)

where

Ndays = number of days in a month.

Npersons = number of persons served by the water heating system. Vpereon = Volume of hot water required per person.

The volumetric consumption, V, varies considerably from person to person and from day to day. It has to do with the habits of the users, the weather conditions of a locality, and various socioeconomic conditions. It can be estimated by considering the hot water use for various operations. Typical operations and consumption for residential usage are given in Table 5.6. More details and other applications, such as water consumption in hotels, schools, and so forth, can be found in the ASHRAE Handbook of Applications (ASHRAE, 2007).

In addition to the quantities shown in Table 5.6, hot water is consumed in automatic dish washing and clothes washing, but these quantities of hot water are produced by the washer with electricity as part of the washing process.

By using the data shown in Table 5.6 for a four-person family and normal daily tasks consisting of two food preparations, two manual dish washings, one shower for each person, and two face or hand washings per person per day, the low, medium, and high demand values in liters per person shown in Table 5.7 can be obtained. The maximum consumption case is where the shower for each person is replaced with a bath for each person per day.

Table 5.7 Hot Water Daily Demand for a Family of Four Persons in Liters per Person

Guideline

Low

Medium

High

Normal consumption

26

40

54

Maximum consumption

66

85

Estimate the hot water energy demand for a family of four, with medium normal consumption, cold water mains supply of 18°C, and water distribution temperature of 45°C.

Solution

According to Table 5.7, the consumption per day per person is 40 L. Therefore, the daily demand, V, is 160 L/d or 0.16 m3/d. From Eq. (5.65),

In hourly simulations, the hourly distribution of hot water demand is required. Although the hot water demand is subject to a high degree of variation from day to day and consumer to consumer, it is impractical to use anything but a repetitive load profile. This is not quite correct during the summer period, when the consumption pattern is somewhat higher. However, during this period, the temperature requirement for hot water is not as high as during winter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. The demand profile usually used in hour simulations is the Rand profile, illustrated in Figure 5.31. This assumes a daily hot water consumption of 120 L at 50°C for a family of four (30 L/person).

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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