8.2. Building Integrated Solar Energy Technologies

8.2. Building Integrated Solar Energy Technologies

Solar energy conversion is a large topic. The key technologies to mention here include:

• Photovoltaics (PV, optoelectronic systems) - convert solar visible radiation into electricity;
• Concentrating Solar Power (CSP, solar thermal, or optocaloric systems) - convert solar thermal radiation into electricity;
• Solar heating systems - utilize solar heat (concentrated or not) without conversion.

You can learn these technologies in depth, taking some of the courses in the Solar Energy Option of RESS. Here in this lesson, we will turn to one of the application examples of PV technology to study the factors that affect the practical implementation of solar panels in buildings. But before doing that, let us review the basic principles by which photovoltaic systems operate.

PV Basics

Photovoltaic (PV) technology is one of the ways to convert solar resource into usable energy - specifically sunlight to electricity. Photovoltaic conversion is enabled by certain semiconductor materials, which have a property to generate electric current when they absorb incident photons. The physics of the photovoltaic effect can be schematically envisioned as three step process [Brownson, 2014]:

1. absorption of light;
2. generation of charge carriers (electrons and holes);
3. separation of charge carriers (so that they can perform work).

This effect is physically realized in certain semiconductor materials  - for example, monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide and some others, all of which can be potentially used for making the PV cells. 

Watch This Video as an introduction to PV cell technology and the photovoltaic effect.

This first short video (2 min) provides an animated illustration of the photovoltaic effect:

Video: Energy 101: Solar PV (2:00)

Next, let us gather some more details on the structure, functions, parameters of the photovoltaic materials.

PV Application in Buildings

Current applications of solar photovoltaics are adapted to the following scales:

• portable (small electronics - e.g., calculators, computers, etc.);
• distributed power generation (homes, isolated equipment);
• utility scale power (solar farms).

In the scale-up process, the single PV cells are combined to modules, and modules can be arranged into arrays:

In this lesson, we choose one of the applications – the building integrated photovoltaics (BIPV) – for more detailed consideration as an interesting example of sustainable technology implementation.

The building-integrated photovoltaics (BIPV) are multifunctional materials that are both structure-supporting and power-generating at the same time. There are some advantages and challenges associated with this technology, which are summarized in table below:

Table 8.1. Advantages and challenges of building integrated photovoltaics

Advantages	Challenges
Lower cost due to multi-functional nature of materials and due to more efficient design. Currently, PV contributes 0.5% to national energy demand; DOE expects it to reach 50% (!) with BIPV implemented.	System orientation, weatherability, durability are the factors that affect the system performance. Those parameters may be not easy to optimize if it is BIPV rather than add-on PV.
Improved aesthetics: panels are produced as facade lining, roof tiles, slates, shingles, windows (glazing, etc.).	BIPV modules can be heavier than regular building elements. Because of potential overload, new building standards and codes need to be in places to regulate this integration.
Thin-film PV can be applied directly to conventional building materials because the flexible and light.	BIPV modules are costly compared to regular add-on PV due to their multifunctional value, but that is not always recognized. BIPV is not mass-produced, but rather custom made, and that adds to the price.
	Social acceptance: not many see long-term value because PV technology is thought to be soon outdated. There should be more acceptance from contractors and end-users alike.

While the main function of a conventional PV system is efficient power generation, the building integrated PV systems, which become the components of the building envelope must satisfy a number of additional requirements, such as:

• appearance: color, image, size;
• weather-tightness;
• wind loading;
• durability and maintenance;
• safety during construction and in use (fire, electrical, structure stability);
• cost.

In order to address all the necessary factors, the PV integration should be discussed early in the beginning of the design process. There are following PV integration options [Robert and Guariento, 2009]:

(a) Shading systems

These include louvers, either horizontal or vertical, which may be mounted outside the building over windows or balconies. Their function is to shade the windows from excessive light. The main issue with these structures is to make them resistant to wind load and easily accessible for maintenance. These subscreens can be made adjustable to maximize the sunlight gain.

(b) Rainscreen systems

Cladding panels can be used to protect the load-bearing external walls from rain water (especially masonry and concrete). PV panels can perform the function of such cladding and tiles. These structures are usually vertical. The ventilated cavity between the exterior cladding and the main wall help keep down the operating temperature of PV cells, enhancing their performance and provide space for cables. PV rain screens have moderate power output due to vertical orientation.

(c) Stick-system curtain walls

Curtain walls are used in those buildings that have internal columns or structural steel to support the main loads. Curtain walls are not weight-bearing and their main function is to resist air and moisture infiltration; also, building insulation is often attached to them (warm facade). A popular solution is to build the curtain walls from aluminum and steel framing filled with glass panels. PV panels can be sealed into curtain wall structures in both vision area or opaque area of the facade instead of regular glass.

(d) Unitized curtain walls

The unitized wall segments are pre-assembled at a factory and then are delivered to the building site. The controlled industrial environment provides more precision and quality for sealing the PV panels into the wall framework and making cable connections.

(e) Double-skin facades

Glass facades are often designed as  "double-skin", i.e., when there is significant air space between the internal and external glass walls. This helps to reduce heat transfer losses through the walls. PV modules can be readily integrated into the external facade. Ventilation through the double-skin structure provides valuable cooling on the back of the panels.   

(f) Atria and canopies

For the highest performance, PV are best integrated into horizontal or tilted elements, such as atria and canopies. These structures are usually free from overshadowing and are easy to ventilate. However, these structures may be more prone to heat transfer losses compared to plain opaque or insulated.

Lifecycle Considerations

One of the main reasons why solar PV systems are being developed is their potential to replace fossil fuel combustion in power generation. While economically, fossil fuels are proved to be profitable, there is a sustainability concern (coal, oil, and gas will run out eventually) and there is an environmental concern (combustion pollutes and contributes to the greenhouse effect on earth). The solar PV can potentially address both of these concerns.

Because, unlike fossil fuels, solar energy is virtually unlimited, PV seems to be a perfect solution from the standpoint of energy sustainability. (Sunlight will not run out any time soon).

From the standpoint of environmental protection, the main question is whether or not the environmental benefit of reducing CO₂ emissions achieved through the use of the solar systems will offset the impact of possible toxic emissions from the manufacturing of the solar modules and potential land pollution from the disposal of those systems at the end of their lifetime.

Let us make some rough estimations of the environmental benefit of PV technology. Consider the following example.

The above example demonstrates the scale of the benefit. Now, let us look at the other side of the medal. The accelerated growth of PV system manufacturing and use will have to become a major market and also should have infrastructure for maintenance and disposal of vast amount of PV-related components. While PV panels are considered are relatively durable with a long projected service life, high volume annual turnover of such systems can be expected. Here, we can do another estimation exercise.

The above-illustrated issue of waste disposal associated with the proliferation of the current PV technology calls for further development of solar cell design. The following improvements will be needed:

• development of thin, low volume solar elements;
• increasing efficiency of energy conversion to minimize the size of systems;
• designing cells for recyclable turn-over and conservation of valuable materials;
• using low-toxicity chemicals and materials in manufacturing;
• creating infrastructure for lifecycle management.