The human body’s comfort is essentially determined by three environmental factors: temperature, relative humidity, and air motion. The single most significant metric of comfort is the temperature of the surroundings. An extensive study on human subjects is carried out to discover the “thermal comfort zone” and the circumstances under which the body feels comfortable in an environment. Most regularly dressed persons relaxing or performing light labor feel comfortable in the operating temperature range of 23 to 27C (approximately, the average temperature of air and surrounding surfaces). This range is 29 to 31 degrees Celsius for naked persons. Relative humidity has a significant impact on comfort because it measures the ability of air to absorb moisture and consequently impacts the amount of heat a person can escape through evaporation. High relative humidity delays heat rejection through evaporation, particularly at high temperatures, whereas low relative humidity accelerates it. The ideal relative humidity level is between 30 and 70 percent, with 50 percent being the most suitable. Most people do not feel hot or cold in these settings, and the body does not need to engage any of its defensive systems to maintain normal body temperature.

Excessive air motion or draft, which produces unwanted local cooling of the human body, is another issue that has a significant impact on thermal comfort. Many people consider drafting to be the most unpleasant aspect of workplaces, vehicles, and airplanes. Persons wearing indoor attire and doing light sedentary work are more likely to feel draft discomfort, whereas people with high activity levels are less likely to feel draft pain. Air velocity should be kept below 9 m/min in the winter and 15 m/min in the summer to avoid draft discomfort, especially when the air is chilly. Low air motion is preferable since it eliminates the warm, damp air that accumulates around the body and replaces it with fresh air. As a result, air motion should be forceful enough to remove heat and moisture from the body’s proximity, yet delicate enough to go undetected. High-speed air movement is also unpleasant outside. Because of the chilling impact of air motion, an environment at 10°C with 48 km/h winds feels as cold as an environment at 7°C with 3 km/h winds (the wind-chill factor).

To eliminate discomfort induced by nonuniformities such as drafts, asymmetric thermal radiation, hot or cold flooring, and vertical temperature stratification, a comfort system should deliver uniform conditions throughout the living space. The cold surfaces of wide windows, uninsulated walls, or cold products create asymmetric thermal radiation, as do the warm surfaces of gas or electric radiant heating panels on the walls or ceiling, solar-heated masonry walls or ceilings, and warm machines. Asymmetric radiation produces pain because it exposes various sides of the body to surfaces at varying temperatures, resulting in unequal heat loss or gain from radiation. Though a person’s left side is exposed to a chilly window, for example, he or she will feel as if heat is being drained from that side of the body. Radiant temperature asymmetry should not exceed 5 degrees Celsius in the vertical direction and 10 degrees Celsius in the horizontal direction for thermal comfort. Radiation asymmetry can be reduced by correctly sizing and installing heating panels, utilizing double-pane windows, and providing enough insulation on the walls and roof.

Direct contact with cold or hot floor surfaces produces localized foot pain. The temperature of the floor is determined by its construction (whether it is directly on the ground or on top of a heated room, if it is built of wood or concrete, whether insulation is used, and so on) as well as the floor covering used, such as pads, carpets, rugs, and linoleum. Most individuals find a floor temperature of 23 to 25 degrees Celsius to be pleasant. For persons who wear shoes, floor asymmetry is irrelevant. Using radiant heating panels instead of cranking up the thermostat is an efficient and cost-effective approach to raising the floor temperature. Temperature stratification in a room, which exposes the head and feet to differing temperatures, is another nonuniform situation that causes pain. The temperature differential between the head and foot levels should not exceed 3 degrees Celsius for thermal comfort. Destratification fans can help to reduce this impact.

It should be mentioned that no temperature climate is suitable for everyone. Some individuals will be upset regardless of what we do. The thermal comfort zone is calculated using a 90% acceptance rate. That example, a place is considered comfortable if just 10% of the inhabitants are unhappy with it. Metabolism slows with aging, although this has no influence on the comfort zone. According to research, there is no discernible difference in the locations favored by elderly and young people. Experiments have also shown that men and women enjoy nearly identical environments. Women’s metabolism rates are significantly lower, although this is offset by their slightly lower skin temperature and evaporative loss. Furthermore, there is no major difference in the comfort zone from one region of the planet to another or from winter to summer. As a result, the same thermal comfort conditions may be used in every season across the world. Furthermore, people cannot acclimate to favor various levels of comfort.

This is actually part-2 of the HUMAN BODY AND THERMAL COMFORT series. part-1 is given in the link below.

Part-1: UNDERSTANDING THERMAL COMFORT AND THE HUMAN BODY.

Follow our youtube channel. and Facebook page.

Spread the love

The phrase air-conditioning is commonly used to suggest cooling, but it also refers to the process of conditioning the air to the desired level through heating, cooling, humidifying, dehumidifying, cleaning, and deodorizing. The purpose of a building’s air-conditioning system is to ensure total thermal comfort for its occupants. As a result, in order to develop an effective air-conditioning system, we must first understand the thermal features of the human body.

Cells are the basic blocks of living beings, like microscopic factories that execute diverse duties essential for organ-ism existence. The human body is made up of around 100 trillion cells with an average diameter of 0.01 mm. Every second, millions of chemical events occur in a normal cell, during which some molecules are broken down, energy is released, and other molecules are produced. Metabolism is the high degree of chemical activity in the cells that keeps the human body temperature at 37.0C while completing the required biological tasks. Metabolism is the process through which meals such as carbs, fat, and protein are burned. Nutritionists often represent the metabolizable energy content of foods in terms of capitalized calories. One calorie equals one kilocalorie = 4.1868 kJ.

The rate of metabolism at rest is known as the basal metabolic rate, and it is the rate of metabolism required to keep a body executing essential biological activities like breathing and blood circulation at zero external activity level. The metabolic rate can also be viewed as the rate at which a body consumes energy. The basal metabolic rate for a typical guy (30 years old, 70 kg, 1.73 m tall, 1.8 m2 surface area) is 84 W. That is, at a rate of 84 J/s, the body converts chemical energy from food (or body fat if the person hasn’t eaten) into heat, which is subsequently dispersed to the surroundings. When performing a vigorous exercise, the metabolic rate increases with the degree of activity and can approach 10 times the baseline metabolic rate. That is, two persons moving vigorously in a room may contribute more energy to the space than a 1-kW resistive heater. While sitting in a classroom, an average male creates 108 W of heat while reading, writing, typing, or listening to a lecture. An average man’s maximal metabolic rate is 1250 W at age 20 and 730 W at age 70. The equivalent rates for women are almost 30% lower. Trained athletes’ maximum metabolic rates can surpass 2000 W.

The table shows metabolic rates per unit of body surface area during various occupations. D. DuBois calculated the surface area of a bare body in 1916. 

where m is the body mass in kilograms and h is the height in meters. Clothing may increase a person’s visible surface area by up to 50%. The metabolic rates in the table are accurate enough for most uses, however, there is significant ambiguity at high activity levels. More precise figures may be obtained by measuring the rate of respiratory oxygen consumption, which ranges from around 0.25 L/min for an average resting man to more than 2 L/min for really strenuous employment. Because the external mechanical effort done by the muscles is so little, the whole energy released during metabolism may be considered to be discharged as heat (in perceptible or latent forms). Furthermore, most activities, such as walking or riding an exercise bicycle, convert work to heat through friction.

Part-5: Thermal comfort air.

Part-4: Adaptations and acclimation mechanisms of Heat in the Human Body.

Part-3: Cold adaptation in humans.

Part-2: Thermal comfort in buildings.

Follow our youtube channel. and Facebook page.

Gain more knowledge here.

Spread the love

What is Electrical load?

Any electrical device people desire to utilize in their homes or offices is referred to as a load. You must decide precisely what loads you want to run and how long you intend to run them before you size a PV system. Throughout the design process, all of your subsequent calculations will be based on this data.

Strategies for designing PV systems step by step are presented solely on a single page.

Calculation:

In order to start the process of sizing all the necessary components, you must first determine how much energy each load (both AC and DC) consumes. Even while going through the load analysis may seem like a hassle, if you don’t spend the time estimating each load’s energy consumption, the installed system will either be significantly undersized or oversized for your requirements. Both scenarios are a waste of time and resources.

You need to know the wattage of each AC load, how long it runs for each day, and how many days a week it is in use in order to calculate the energy usage of each AC load in kilowatt-hours (kWh). Some loads might only run a few times each week, while others might run every day.

A regular pattern of energy use (average daily energy consumption) can be established by averaging out the loads over the course of a week. Use the equation shown below:

Energy (in watt-hours) = (Watts × Hours per day × Days per week) ÷ 7 days per week

Include all of the watts drawn when evaluating your weekly energy needs. For instance, when estimating energy use for lighting, consider all of the lights that will be on at once rather than just one.

Here’s another example: imagine your washing machine isn’t used every day. You would get at a result that is larger than usual if you calculated the washing machine’s energy consumption based on the days it operates. Instead, by averaging the washing machine’s energy usage over the course of a week, you arrive at a daily energy consumption that is marginally higher than the actual amount on days when the machine isn’t in use and marginally lower than the actual amount on days when it is. This yields a week-long estimate that is fairly accurate.

Take a 175 W washing machine and run it for 45 minutes. If it runs four days a week, multiply the power draw by the number of hours to get the average daily energy value:

175 W × 0.75 hrs = 131 Wh, or 0.131 kWh

The washing machine operates four days a week and consumes 0.131 kWh every day. Therefore, the average daily energy consumption is:

131 Wh × 4 days ÷ 7 days = 75 Wh per day.

Tracing the specific energy consumption levels for all the loads your PV systems need to supply might be challenging. To make it easier to keep track of all the calculations, I advise utilizing a spreadsheet. Additionally, it is simpler to make significant modifications once you have a spreadsheet. As in Figure, keep your load-analysis spreadsheet straightforward. A few loads that will be powered by a PV system are shown in this load chart.

The itemized loads are shown in the first column, and the number of each item is shown in the second column. The following two columns provide the estimated number of hours per day and days per week that each load is expected to run for. The third column shows the power demand (in watts) of a certain load. The final column calculates the average daily energy consumption of each load by multiplying the total run watts by the number of hours per day multiplied by the number of days per week and then dividing by seven. The second-to-last column shows the number of watts each load draws multiplied by the number of items.

Strategies for designing PV systems step by step are presented solely on a single page.

To understand the load chart well, take a look at the television row. Your family watches TV on a nice new big-screen TV that uses 200 W while it is on and is left on for at least 4 hours. The TV requires 200 W for 4 hours or 800 Wh. assuming that everyone in your family watches TV 6 days a week. In order to calculate the average daily energy usage for the week, multiply the 800 Wh by the 6 days per week that your family actually watches TV and divide the result by the 7 days per week. Doing so puts the average daily energy consumption for the TV at 686 Wh, or 0.686kWh.

You must include in the energy requirements of any DC appliances you want to utilize, such as lighting or refrigerator. You must account for the efficiency losses caused by converting the DC in the PV modules to AC for the AC loads; therefore, you calculate this consumption in the same manner that you do for AC loads, but separate the values in your tables. The overall energy consumption of all the loads can be calculated by adding the two figures after accounting for efficiency losses.

Unless sophisticated energy management techniques are applied while configuring the inverter, the inverter itself is an ongoing load. The inverter’s idle draw can have a considerable impact on the rest of your system sizing and the system’s overall performance, even while this load isn’t particularly huge or continuous. By consulting the specification (spec) sheet offered by the manufacturer, you can determine the precise amount of energy an inverter consumes while it is not in use. Simply take a look at the specifications of the inverters you plan to use most frequently and calculate each one’s power requirements.

A PV system designer must know A few electrical concepts, like series and parallel wiring, a set of electrical circuit laws, and IV curves must be understood before you can construct your system. We’ll go over each fundamental principle before giving you a few straightforward applications that you may use in your own analyses. Whether you’re designing for string inverters or microinverters, these fundamental concepts are crucial.

Strategies for designing PV systems step by step are presented solely on a single page.

There are two wiring configurations utilized for various components of a PV system: series wiring and parallel wiring. The series strings are made by wiring individual modules together in series. After that, the series strings are connected in parallel before being fed to the rest of the system. Both off-grid and grid-tied systems using string inverters are covered by this. Off-grid users will also wire the battery bank in series and parallel. With microinverter systems, parallel wiring is not a big deal. When you connect the positive (+) lead of one module to the negative (-) lead of the next, you are wiring modules in series. You have one additional negative lead and one additional positive lead at the end of this string.

Series wiring has the effect of increasing voltage while maintaining the same amperage. When 10 modules are connected in series and each has an output rating of 30 volts at 8 amps, the string as a whole will have a voltage rating of (30×10) 300 volts at 8 amps.

When wiring series strings of modules in parallel, you must connect the extra positive and negative leads of the series strings together. Parallel wiring has the effect of maintaining the same voltage while increasing the amperage. For instance, if you connect two of the aforementioned strings (each rated at 300 volts and 8 amps) in parallel, the circuit as a whole will have a rated output of 300 volts and 16 amps. The rated power output of this array is 300 x 16 = 4,800 watts (remember that volts x amps equal watts).

Due to equipment compatibility and capability, you should connect modules in series and series strings in parallel rather than the other way around. PV modules typically produce a relatively high current and low voltage (around 30 volts DC) (typically about 8 amps DC). Contrast that with a typical home electrical circuit, which uses 120 volts AC and can only handle 15 or 20 amps AC at most. The voltage is increased without increasing the current when the modules are wired in series. Additionally, 200 to 400 volts and 10 to 40 amps are commonly supported by string inverters. These voltage and current ranges can be reached by the PV array by wiring modules in series and series strings in parallel.

Rules for DC Electrical Circuits:

A PV system designer must know Basic DC electrical circuits wired in series and parallel are governed by three electrical circuit principles.

Rule 1: The voltages increase while the amperage remains constant when modules are coupled in series.

Rule 2: The voltages of each of the strings must match when a series of strings of modules are wired in parallel.

Rule 3: The total amperage of the strings increases but the voltage of each string remains constant when series strings of modules are wired in parallel.

When determining the output values for your modules and strings and making sure the values are compatible with your inverter’s input needs and standards, you will follow these three guidelines. They also have an impact on how you build the module strings, although string inverter and microinverter systems are subject to distinct restrictions.

String inverters:

Every series string in an array must have an equal number of modules in order to comply with rule 2. If you know that your array will have 18 modules in total, for instance, you can wire them into two strings of 9 modules or even three strings of 6 modules, but you shouldn’t wire them into one string of 10 and one string of 8. If you have uneven strings, one will be functioning over its optimal value and the other will be operating below its optimal level since rule 2 requires that both strings of the parallel circuit have the same voltage. As a result, neither string will be using its ideal voltage level. With the help of the discussion of I-V curves that follow, you will comprehend this better.

Microinverters:

The aforementioned guidelines don’t apply since microinverters convert from DC to AC at the module. As a result, you don’t need to bother about assigning equal numbers to each group of your modules. One string of 10 modules, one string of 8, or even two strings of 7 and one string of 4 are acceptable configurations without degrading performance, for instance. You must abide by the manufacturer’s restriction on the number of microinverters that can be linked together. Some arrays may consist of a single string since some microinverters can be connected in long strings of 20 or more modules. However, the majority of microinverters have lower limits, leading to arrays with two or more strings. Branch circuits are the official name for module groupings in microinverter systems, possibly to avoid confusion with string-inverter strings. Both words describe a set of modules connected in series. The maximum number of units per string for microinverter systems must not be exceeded by strings (branch circuits).

IV Curve:

A PV system designer must know The relationship between current, or amperage (I), and voltage (V), in an electrical device such as a solar cell or PV module, is depicted graphically by an I-V curve. An I-V curve illustrates how variations in either the current or the voltage affect electrical output since electrical power (P, expressed in watts) is a product of current and voltage:

P (power) = I (current) × V (voltage) and 1 watt = 1 amp × 1 volt

The I-V curve of a single PV module shows that the current is zero when the voltage is at its maximum (in this case, 40 volts). This equals 0 watts of power (40 volts x 0 amps = 0 watts). Conversely, when the voltage is at 0 and the current is at its maximum (9 amps), there is no power generated (9 amps x 0 volts = 0 watts). This indicates that you must balance the current and the voltage in order to generate usable electricity.

A PV system designer must know Voltage can be thought of as the pressure that pushes electricity across an electrical circuit. Consider the pressure of water in a garden hose. The flow of electrons in a circuit, which is referred to as current, is a function of pressure. Consider the quantity of water flowing via a garden hose. The positive (+) and negative (-) output wires of a photovoltaic module, when connected together, produce a short circuit with maximum current (high flow), but zero voltage (no pressure), as there is no resistance to the flow. The point where the I-V curve meets the Y (current) axis is known as the short-circuit current value (Isc) on the module’s specification sheet. When the two wires are disconnected, an open circuit is formed with maximum voltage (high pressure), but no current (no flow). Even if the solar cells are working, there is nowhere for the electrons to travel, thus no power is produced. The point on the IV curve where the curve meets the X (voltage) axis is identified as the open-circuit voltage value (Voc) on the module’s specification sheet.

Strategies for designing PV systems step by step are presented solely on a single page.

Every point on this curve has a power value (in watts) that is equal to the current (amps) times the voltage (volts) at that point. When the IxV product, or the product of the values of the current and voltage along the IV curve, is largest, this is when you get the most electric power. The maximum power point (MPP, or Pmax ) is the name given to this sweet spot. This is always close to the IV curve’s “knee”; in this given curve above, P is at roughly 33 volts and 7.6 amps. This is the same as the module’s rated output (33 volts x 7.6 amps = 250.8 watts), which is also where we want the module to function.

DC-AC inverters (both string inverters and microinverters) sense the current and voltage in a module’s or string’s circuit and then modify the circuit’s level of resistance to alter the voltage and current as necessary to always hit the MPP. The inverter modifies resistance to modify the voltage and maintain the MPP at the lower current value if the current decreases. In order to run at the MPP, the inverter once again changes the resistance as the current increases, altering the voltage and raising the output. No matter the weather outside, the homeowner always receives the highest power output from their PV system. A PV system designer should thanks to this ongoing MPPT optimization technique.

How to use PVWatts to figure out your PV system size

Take a moment to visit the PVWatts calculator online for fun. Click  “Go” after entering your city and state. Choose the map position that is nearest to your home on the following page, or simply continue with the default location chosen for your city, and then click the orange arrow that says “Go to system info.” Then click the “Go to PVWatts results” arrow on the following page to finish. How much AC electricity you can generate annually at your location with a 4 kW (DC) system facing due south is indicated at the top in large type. The calculator already has the system size and other pre-populated default values.

Strategies for designing PV systems step by step are presented solely on a single page.

Even if your system might not be 4 kW or face due south, this 60-second exercise will nonetheless demonstrate how simple it is to start using PVWatts. There is no cost, no need to register, and no advertising.

PVWatts appears to be quite straightforward, but there is a lot more you can do to enhance accuracy. You may simply switch back and forth between the pages, plug in various data, and immediately check the outcomes because it is made for trial and error. Your particular data can be entered on the “System Info” page.

For example, let’s consider we have an average annual energy requirement of 6310 KWhrs/year. Which we calculated earlier. If you want to learn how we have calculated it you can always learn from here.

Now we need to determine the size of our PV array that will be able to meet our annual energy requirement i.e. 6310 KWhrs/year based on a desired location and other factors. For this, we need to put a value in the DC System Size (kW) field of the PVWatts calculator which will return a result of 6310 KWhrs/year. You can always play around with the DC System Size (kW) value to obtain your required annual energy requirement i.e. 6310 KWhrs/year based on your location and other factors.

In our case, our desired location is given below:

In our case DC System Size (kW) value and other considerable values in given as such to obtain our required annual energy requirement i.e. 6310 KWhrs/year is given below. We entered 4.483KW in DC System Size (kW) field and 23 and 205 in Tilt(deg) and Azimuth(deg) fields respectively. Other parameters are kept intact for simplicity.

It is time to get our desired Result

Boom!! we got our desired result which is exactly the same as our annual energy requirement i.e. 6310 KWhrs/year.

The result simplifies the fact that if we have an energy demand for a year is 6310 KWhrs or we need to use 6310 KWhrs of energy for a year then we need to install a PV array having a size of 4.483KW. In other words, a 4.483 KW PV system will provide 6310 KWhrs of energy for a year.

Strategies for designing PV systems step by step are presented solely on a single page.

Follow our youtube channel. and Facebook page.

Spread the love