Active Vs. Passive Solar Power (Helpful Guide)

When converting your home to solar power, there are several factors to consider, and the entire process can be quite overwhelming. We often believe that “solar is solar,” but did you know that we have active and passive solar power? If not, allow me to elucidate you as you delve into active vs. passive solar power.

Active and passive solar power function as a heating source for your house/building. Passive solar uses the orientation of your house to heat masonry, which releases heat into the rest of the house. Active solar heats a liquid or air in collectors, which heat the building.

Active and passive solar power focus on harnessing the sun’s radiant energy to reduce heating costs in homes and buildings. But how is this energy captured, transferred, and used? Are they sustainable, and what are some benefits and drawbacks?

Active Vs. Passive Solar Power

An Overview Of Solar Power: Source, Function, And Conversion

In its most basic form, solar power takes the energy provided by the sun (in the form of heat and light) and converts it to a usable energy source.

We can harness solar energy in two different ways. 

  1. Using the sun’s energy to heat salts, sand, water, and other storage materials. These materials are used to heat other water sources (mostly). This type of solar energy enhancement is often called concentrating solar-thermal power. 
  1. Sunlight is directly converted into electricity through photochemical reactions in solar panels. This type of solar power usage is through photovoltaics. Sunlight strikes a solar panel which causes electrons to loosen inside the panel. These free electrons move, creating a current and thereby producing electricity.

How Does Active And Passive Solar Power Tie In?

Both active and passive solar power convert solar energy into heat. This heat is either stored for future use or used immediately. The method in which they do this, however, is what differs. 

Below I compare active and passive solar power in harvesting, usage, and pros and cons.

A. Active Vs. Passive Solar Power: Harvesting Techniques

1. Active Solar Power Harvesting

harvesting
  • When actively utilizing the sun’s energy, we use panels (collectors) or other devices to “catch” the sun’s energy and transform it into a usable form and transfer it. This method of acquiring solar energy is through electrical or mechanical methods.
  • Active solar heating involves harnessing the sun’s energy through heating air or a liquid. This energy is either used immediately or transferred to a storage device. 
    • Liquid-based systems work similarly to a radiator in a car, if only with hot water instead of cold.
    • Collecting panels are filled with water, antifreeze, or some other liquid heated by the sun. 
  • This heated liquid transfers heat when a control unit causes the liquid to circulate through the collectors.  
  • From the collectors, the heated water moves to a storage tank for later use or a heat exchanger/endpoint for immediate use. 
  • Air-based systems work similarly to liquid ones but instead use air as the heating medium. Once heated, the air is used to heat a room directly, or the hot air is used in conjunction with a heat recovery ventilator or air coil.

2. Passive Solar Power Harvesting

Active Vs. Passive Solar Power

Passively harvesting the energy from the sun is related to the design layout of a building. By focusing on certain features, the sun’s light energy is maximized.

Some of these design features include:

  • An aperture. An aperture collects/allows light to pass through into the building. These are usually south-facing windows (or at least within 30 degrees). These glass “openings” need to be free of obstructions like trees and buildings, especially between 09:00 am and 3:00 pm. 
  • An absorbent surface/material. This surface catches the sun’s energy and retains it while simultaneously transferring it to the layers below/behind it. These are usually dark surfaces of a hard material, like stones, bricks, or concrete. Sometimes water tanks are also used.
  • A Thermal mass. This thermal mass is the layer that lies beneath the absorbent material (often, it is the same thing). The purpose of this mass is to store the heat energy from the sun. Thermal masses are usually stone, brick, concrete, or other heat-retaining materials. 
  • A means of distribution. Once the heat is stored in the thermal mass, it needs to be distributed to the rest of the building. Depending on the design, the distribution employed could rely on natural heat distribution methods (convection, conduction, and radiation). 
  • A way to control the system. There is such a thing as too much heat, especially during the summer months. Roof overhangs or blinds create shade, electronic sensing devices, thermostats connected to fans, and vents and ducts that restrict/remove heat.
  • Passive solar harvesting works with:
    • direct gain – the thermal mass is located in the house, and heat energy is collected then re-radiated into the building),
    • or indirect gain – a Trombe wall (the sun’s energy is intercepted by the thermal mass which then uses convection to heat the interior of the building. 

B. Active Vs. Passive Solar Power: Uses

1. Active Solar Power Uses

water tanks
  • Heating/cooling the air (solar space heating). Usually used in a room. This heating is achieved through the use of both heated air and liquids. A thermal mass, rock bin, or other storage unit stores the heat and distributes it via ducts and blowers when using air.
    • Liquid-based heating is used to heat rooms through underground pipes, pumps, radiant slabs, central forced air, and hot water baseboards.
  • Solar water heating. The liquid is circulated through the building with pumps, water, or heat transfer. The two methods are:
    • Direct method is where water is heated in the solar collectors (panels), pumped into storage tanks, and used as needed. 
    • Indirect method involves heating a liquid in the collectors. This heated water is pumped to storage tanks inside a coil/radiator, where it heats usable water.
  • Solar pool heating. The process is similar in solar water heating, except there is no storage tank. Hot water from the collectors moves through pipes straight into the pool.

2. Passive Solar Power Uses

  • Used as features in the house and other building designs to maximize solar energy in a “natural” way. 
  • Used to heat or cool an area. 
  • Stored warmth is circulated through vents and other means to heat an area. 
  • Emitting the cooler air. The process is similar when cooling an area, except instead of heat, the thermal mass stores the cold, which then cools down a building (like opening a fridge). 

C. Active Vs. Passive Solar Power: Benefits And Drawbacks

The table below compares some of the benefits and drawbacks of active and passive solar power.

BenefitsDrawbacks
Active solar powerMore control of the amount of heat entering a room.

A sustainable source of heating.

Additional collects are connected to increase the heat generated.

Air-based collectors do not freeze.
Very expensive to establish and maintain. 

Antifreeze liquids are potentially hazardous.

Liquid-based collectors can potentially freeze.

Inefficiencies and heat loss limit the efficacy. 

To heat buildings effectively, the temperatures required are between 160° and 180°F.
Passive solar powerLow operating costs.

A sustainable source of heating.

No additional technology is required for it to work.

No harmful chemicals are involved in the process. 
Very little control over temperatures, as the sun and house design, dictates what is achievable.

The efficacy is limited, especially by the weather.

Overheating is a likely concern in hotter areas.

Although both are sustainable sources of heat energy, neither is truly efficient in energy conversion.

Passive systems are cheaper than active but are limited by the orientation of your house. I feel that a passive system has the potential to be better if all the factors work together and you find the “sweet spot.” 

The reality is that technological developments have improved over the last couple of decades in enhancing solar energy (both in generating electricity and heating). There is, however, still a long road ahead before solar power and green energies, in general, replace the efficiency and wide-scale use of fossil fuels.

Conclusion

Although they serve the same function of heating a building, active and passive solar power differ in achieving this goal. Passive solar power allows the sun to heat a thermal mass and relies on the orientation of the house to achieve maximum heat. Active solar power involves heating water or air, heating other usable water or buildings. Both are sustainable sources of heating, but both are limited in their efficacy. 

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