A solar-assisted heat pump (SAHP) is a machine that combines a

heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing h ...

and thermal solar panels and/or PV solar panels in a single integrated system. Typically these two technologies are used separately (or only placing them in parallel) to produce

hot water Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...

. In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump's evaporator. The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way. It is possible to use any type of solar thermal panel (sheet and tubes, roll-bond, heat pipe, thermal plates) or

hybrid Hybrid may refer to: Science * Hybrid (biology), an offspring resulting from cross-breeding ** Hybrid grape, grape varieties produced by cross-breeding two ''Vitis'' species ** Hybridity, the property of a hybrid plant which is a union of two dif ...

(

mono Mono may refer to: Common meanings * Infectious mononucleosis, "the kissing disease" * Monaural, monophonic sound reproduction, often shortened to mono * Mono-, a numerical prefix representing anything single Music Performers * Mono (Japanese b ...

polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

thin film A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...

) in combination with the heat pump. The use of a hybrid panel is preferable because it allows covering a part of the electricity demand of the heat pump and reduce the power consumption and consequently the

variable cost Variable costs are costs that change as the quantity of the good or service that a business produces changes.Garrison, Noreen, Brewer. Ch 2 - Managerial Accounting and Costs Concepts, pp 48 Variable costs are the sum of marginal costs over all un ...

s of the system.

Optimization

The operating conditions' optimization of this system is the main problem, because there are two opposing trends of the performance of the two sub-systems: by way of example, decreasing the evaporation temperature of the

working fluid For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...

increases the

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a he ...

of the solar panel but decreases the performance of the heat pump, and consequently the COP. The target for the optimization is normally the minimization of the electrical consumption of the heat pump, or

primary energy Primary energy (PE) is an energy form found in nature that has not been subjected to any human engineered conversion process. It is energy contained in raw fuels, and other forms of energy, including waste, received as input to a system. Pr ...

required by an

auxiliary boiler There have been a vast number of designs of steam boiler, particularly towards the end of the 19th century when the technology was evolving rapidly. A great many of these took the names of their originators or primary manufacturers, rather than a m ...

which supplies the load not covered by

renewable source Renewable energy is energy that is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, the movement of water, and geothermal heat. Although most renewable energy ...

Configurations

There are two possible configurations of this system, which are distinguished by the presence or not of an intermediate fluid that transports the heat from the panel to the heat pump. Machines called indirect- expansion mainly use

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

as a heat transfer fluid, mixed with an antifreeze fluid (usually

glycol A diol is a chemical compound containing two hydroxyl groups ( groups). An Aliphatic compound, aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common ...

) to avoid

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

formation phenomena during winter period. The machines called direct-expansion place the refrigerant fluid directly inside the hydraulic circuit of the thermal panel, where the

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...

takes place. This second configuration, even though it is more complex from a technical point of view, has several advantages: * a better transfer of the heat produced by the thermal panel to the working fluid which involves a greater thermal efficiency of the evaporator, linked to the absence of an intermediate fluid; * presence of an evaporating fluid allows a uniform temperature distribution in the thermal panel with a consequent increase in the thermal efficiency (in normal operating conditions of the solar panel, the local thermal efficiency decreases from inlet to outlet of the fluid because the fluid temperature increases); * using hybrid solar panel, in addition to the advantage described in the previous point, the

electrical efficiency The efficiency of a system in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a fractional expression), typically denoted by the Greek small letter eta (η – ήτα). : ...

of the panel increases (for similar considerations).

Comparison

Generally speaking the use of this integrated system is an efficient way to employ the heat produced by the thermal panels in winter period, something that normally wouldn't be exploited because its temperature is too low.

Separated production systems

In comparison with only heat pump utilization, it is possible to reduce the amount of electrical energy consumed by the machine during the weather evolution from winter season to the spring, and then finally only use thermal solar panels to produce all the heat demand required (only in case of indirect-expansion machine), thus saving on variable costs. In comparison with a system with only thermal panels, it is possible to provide a greater part of the required winter heating using a non-fossil energy source.

Traditional heat pumps

Compared to

geothermal heat pump A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that uses a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through ...

s, the main advantage is that the installation of a piping field in the soil is not required, which results in a lower cost of investment (drilling accounts for about 50% of the cost of a geothermal heat pump system) and in more flexibility of machine installation, even in areas in which there is limited available space. Furthermore, there are no risks related to possible thermal soil impoverishment. Similarly to

air source heat pumps An air source heat pump (ASHP) is a type of heat pump that can absorb heat from outside a structure and release it inside using the same vapor-compression refrigeration process and much the same equipment as air conditioners but used in the opp ...

, solar-assisted heat pump performance is affected by atmospheric conditions, although this effect is less significant. Solar-assisted heat pump performance is generally affected by varying

solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...

intensity rather than

air temperature Atmospheric temperature is a measure of temperature at different levels of the Earth's atmosphere. It is governed by many factors, including insolation, incoming solar radiation, humidity and altitude. When discussing surface air temperature, the ...

oscillation. This produces a greater SCOP (Seasonal COP). Additionally, evaporation temperature of the working fluid is higher than in air source heat pumps, so in general the coefficient of performance is significantly higher.

Low temperature conditions

In general, a heat pump can evaporate at temperatures below the ambient temperature. In a solar-assisted heat pump this generates a temperature distribution of the thermal panels below that temperature. In this condition thermal losses of the panels towards the environment become additional available energy to the heat pump. In this case it is possible that the thermal efficiency of solar panels is more than 100%. Another free-contribution in these conditions of low temperature is related to the possibility of

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of water vapor on the surface of the panels, which provides additional heat to the heat transfer fluid (normally it is a small part of the total heat collected by solar panels), that is equal to the

latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understo ...

of condensation.

Heat pump with double cold sources

The simple configuration of solar-assisted heat pump as only solar panels as heat source for the evaporator. It can also exist a configuration with an additional heat source. The goal is to have further advantages in energy saving but, on the other hand, the management and optimization of the system become more complex. The geothermal-solar configuration allows reducing the size of the piping field (and reduce the investment) and to have a regeneration of the ground during summer through the heat collected from the thermal panels. The air-solar structure allows an acceptable heat input also during cloudy days, maintaining the compactness of the system and the easiness to install it.

Challenges

As in regular air conditioners, one of the issues is to keep the evaporation temperature high, especially when the sunlight has low power and the ambient airflow is low.

References

External links

* * * {{HVAC Heat pumps Building engineering Energy conversion Energy recovery Energy technology