Just as conventional cogeneration makes use of ‘waste’ heat formed in thermal power generation, a new concept being developed in Turkey will use heat otherwise wasted in solar photovoltaic (PV) systems. Here, Birol Kilkis describes his photovoltaic and heat system, and its trigeneration equivalent.

In today’s photovoltaic (PV) technology, the thermo-electric conversion efficiency decreases with the ambient temperature. Therefore, when the solar radiation reaches a maximum, PV efficiency becomes a minimum. PVT (photovoltaic and heat) systems are designed to capture this heat in a useful manner in order to increase both the energy and exergy efficiency of PV systems, while the PV module remains relatively cool.

This is nothing but cogeneration: PV panels draw energy only from a narrow spectrum of solar energy to generate electric power. The remaining energy, which is mostly in the form of heat, remains unused. The simple but powerful idea behind PVT is to capture a much wider spectrum of the solar energy – to generate both electricity and heat. The total thermal efficiency, namely power and heat generation may exceed 75%.

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Figure 1. Solar trigeneration concept

A new technology has been developed that pushes the PVT idea one innovative step ahead: solar PVTC, namely solar photovoltaic, heat and cold. That is again nothing but trigeneration in its smallest possible size. This has been named nano-sized solar trigeneration, mainly because every component is electronic.

PVTC CONCEPT

The fundamental idea of the patented all-electronic solar trigeneration system is shown in Figure 1.

In spite of its environmental benefits, the problem with PVT is the fact that there is not much thermal load in a building during summer, especially in residences. Instead, cooling loads are dominant. Therefore, the reclaimed solar heat from PVT panels needs to be further converted to cold. This means that excess heat should be converted to cooling effect by an absorption system or by a metal-hydride system. Although these are technically feasible, the system mechanics becomes quite complicated and operation and installation costs may prove to be uneconomic. Instead, an all-electronic version of the concept was invented, in which the system comprises an integrated, sandwiched unit that employs the electro-thermal effect both for heating and cooling.

In the new, so-called PVTC (photovoltaic, thermal, and cooling) system electric power, heat and cold may be simultaneously generated in a useful manner based on micro-electronic comp-onents only. The next step will be a nano-sized trigeneration system in one integrated module.

As illustrated in Figure 1, the solar PV module and the thermo-electric cooling (TEC) module set are interfaced, sandwiched and integrated through a thermal conductive sheet of very high thermal con-ductance. This conductive sheet performs so that the heat, while it is transferred to a proper heat sink at the demand point, cools the PV module and maintains the proper temperature difference across the TEC module.

During typical operation, the cooling enables the solar PV module to generate electric power at its optimum performance level. With a simple control, this solar power may be optimally split between the building’s and the TEC module’s power needs. When the TEC module is exposed directly or indirectly (by a second thermally conducting sheet layer) to the indoor space, it electronically heats or cools the space primarily by thermal radiation and secondly by natural convection through its exposed surface, depending upon the polarity of the DC power supplied by the PV module, which is controllable.

In the cooling mode, the TEC module absorbs heat from the indoor space and transmits it to the same heat conducting sheet between the TEC module and the PV module. Thus this system multiplies the heat gain that may be usefully utilized in the same indoor space or other building zones. If the cooling load is the dominant load, part of this heat may be further utilized in a heat activated cooling system like a metal-hydride system.

During the space heating season in winter, a simple switch of the polarity makes the same TEC module a radiant space heating module. In this case, the heat conducting film may bring the PV heat into the indoor space. The fundamental advantage of this concept is that power, heat and cold generation is combined in the same unit of solar PV surface – which is at premium, especially in densely populated urban districts.

The final result is a decentralized system in every roof. Again, this system is especially valuable in buildings because solar exposure area is at premium and quite limited. The same system may also be adapted to building facades and exterior walls, as depicted in Figure 2. The inclination angle and the spacing of PV modules depend on the geographic location.

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Figure 2. Building integrated version of PVTC

For an incident total solar insolation on a 1 m2 of PVTC panel surface area of 160 W, the breakdown of the useful outputs is given below:

Total solar electric power supply = 32W n 16 W to electric demand (net power supply to the building)

16 W to TEC (for internal use in photovoltaic-thermal-cooling-insulation modules)

TEC cooling capacity = 11.2 W at an indoor air temperature of 22°C

n  Solar heat capacity = 115.2 W at 70°C

n  Total useful output: 115.2 W + 16 W + 11.2 W + heat gain from comfort cooling

n  The first law efficiency of the module under ideal conditions is 142.4/160 = 89%, excluding the heat gain from comfort cooling. If this gain is also included (about 15 W), the coefficient of performance value of the PVTC system during comfort cooling approaches one.

EXERGY EVALUATION

In its simplest definition, exergy is the amount of useful work potential of a given amount or stream of energy source. In other words it is a matter of the quality of a source.

The supply exergy to the PVTC, the exergy from the solar incidence on the PV module is 93.7 W for each module at the given conditions noted above (160 W). The exergy value of the heat generated from the same set is 20.1 W. Add another 2.7 W from the indirect solar gain absorbed by the TEC module from the room. Because electricity is a high quality component of the total energy obtained from the sun, its exergy practically equals the net electric power (16 W).

In the example given above, the cooling exergy from the TEC module is typically 6.6 W. The sum of all exergy values is 45.4 W. Then the rational exergy management efficiency, of the PVTC module is 45.4/93.7 = 0.48.

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Figure 3. Solar goes cogeneration and trigeneration on a nano scale.

The same value for a simple PV system is only 32/93.7, which is 0.34 and its first law efficiency is equal to 0.20 (32/160). Compare this with a first law efficiency of the PVTC module, which is 0.89 (see Table 1). Table 1 summarizes the benefits of solar PV, PVT and PVTC in comparison to a natural gas boiler system in a dwelling connected to the national power grid.

DISCUSSION

Figure 3 graphically compares PV, PVT and PVTC systems, all of which are actually decentralized energy systems for the built environment, especially for buildings. However, starting from the simplest case of the PV system, solar cogeneration and solar trigeneration takes the decentralized power concept (PV) to a truly decentralized trigeneration level on a micro scale, which is heading towards the nano scale.

Figure 3 shows how the waste in a PV system may be recovered to useful work (exergy) in the form of heat and cold. The PHVT (photo-heat-voltaic and thermal) is a case where the TEC module is interfaced with the back side of the PV module in the opposite manner than PVTC in such a manner that the TEC generates additional electrical power from the heat absorbed by the PV module from the sun.

The reject heat is collected in the form of useful heat, as for domestic water heating. A PHVT module almost doubles the total PV efficiency in the same unit surface area. This alternative shows how decentralization may reach new heights in different combinations of PV and TEC technology especially in buildings.

Performance

Figure 3. Solar goes cogeneration and trigeneration on a nano scale.

Parameter

Solar application scenarios

Base scenario

Solar PV

Solar PVT (DHW supply only)

Solar PVTC

Boiler and grid power

First law efficiency

0.20 (max)

0.92

0.89

Boiler: 0.95

Grid power: 0.35

Rational exergy management efficiency

0.34

0.55

0.48

0.06

Fossil fuel savings

100%

100%

100%

-8%

Coefficient of performance, COP

<<1

<1

≥1

Carbon dioxide reduction potential

0.3 units

0.63 units

1.6 units

-1 unit

Function(s)

Power

Power and heat

Power, heat, and cold

Heat

Typical pay-back period*

12-14 years

8 years

5-6 years

never

Table 1. Benefits of decentralized PVT and PVTC compared to PV
*Depends on many factors. These values are only for relative evaluation of scenarios and the base case.

Birol Kilkis is the Energy Engineering Graduate Programme Head, Baskent University, Ankara, Turkey.
Email: birolkilkis@hotmail.com 

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