Updated: Dec 14, 2020
It might seem strange using the terminology Solar Cooling which to many is a new and possibly strange term and one they have never heard of. To make it more confusing then also writing an article about it in the middle of a UK winter, but the fact is Solar thermal technology isn’t new and the benefits and energy savings can be substantial all year round.
However, the process of integrating a solar thermal system into a HVAC refrigeration circuit is new – and it once you fully understand how it works, and proof that over 6,000 installations worldwide can’t be wrong - right! Shows that it offers substantial energy savings no matter what the time of year.
This article is written for the benefit of Clients who would like to consider introducing substantial Energy savings to the Buildings they are involved with but also for those that maybe more sceptical, are more technically minded and want to understand how and why it works.
Benefits of solar thermal cooling include the following:
Reduction in greenhouse emissions due to using the sun’s free energy.
Reduction in overall HVAC/R energy consumption of 30-65%, even when including evenings and winter seasons.
These systems can be used on new or existing HVAC systems that have variable flow capability and compressors able to slow or stage down.
When replacing an existing HVAC/R system, the solar thermal components can just be re-connected into the new system with minor adjustments.
Increase in compressor life expectancy and a reduction in refrigerant losses
No system is too large or too small.
No moving parts
The hotter the sun, the more efficient the system becomes.
The highest efficiencies are achieved during peak hours.
How are these savings being achieved?
The introduction of variable load HVAC/R systems (VRF, inverter, staged, screw, digital scroll, compressors) has made the integration possible. This article demonstrates an overview of the technology and shows proven and tested energy savings by reducing energy consumption of HVAC/R usage by 30-65%.
So, What is solar thermal cooling?
Boasting about the huge savings that we have already achieved across more than 6,000 worldwide installations does not necessarily give us the credibility when discussing the technology with potential new customers.
The due diligence required by the Consulting Engineers, Specifiers, technicians and other engineers and advisors to all clients means not only do they want to see the evidence, they must also understand how the technology works in detail, in most cases prior to even conducting an evaluation.
However, in truth there are many Chiller and VRF systems where the technology isn’t viable or suitable. i.e. Those whose systems which are over specified (more chillers than needed) or simply oversized. Also, where roof space is limited.
The SolarCool technology can only be partnered with variable or modulating systems. Therefore, it’s paramount that the specialists looking to appreciate the technology should hold at least a basic understanding of the variable capacity refrigeration technology.
Essentially, knowledge of the variable flow system should be good enough to allow the reader to park old theories involved in the process of single, fixed-speed compressor technologies, having a fixed volume flow.
Understanding Variable Flow Refrigeration
Our experience shows that a lack of understanding on how the variable system differs from the classic fixed speed system, will almost certainly result in the reader failing to comprehend or appreciate the thermodynamics and/or physics of how solar cooling realises the additional efficiency.
The idea of adding heat to the air conditioning/refrigeration cycle appears to many sceptics to counterproductive and illogical. However, you must remember that most air conditioning systems and chillers are essentially heat pumps that cool buildings by moving heat out of them.
The vapour compression cycle used in most heat pumps is driven by a high-temperature and/or high-pressure refrigerant. To generate these high temperatures and pressures, electricity is used to drive a compressor. Our solar thermal cooling system uses solar energy to reduce the electricity needed to operate those compressors.
The diagram below shows the typical Variable flow vapour compression cycle connected to a solar thermal panel. The thermal collectors are installed on the discharge side (hot gas side) of the compressor. Free solar energy is injected into this hot refrigerant gas, increasing the kinetic energy of the molecules and the velocity and temperature of the refrigerant.
The higher temperature of the refrigerant gas increases the delta temperature (Delta T) between the condenser coils and the outside air, which increases the heat flow out of the system.
The increased heat flow means the gas reaches the equilibrium point (the conversion of the refrigerant from gas to liquid) earlier within the condenser coil, which also improves the effectiveness of the heat transfer and conversion back to liquid.
CYCLE OF REFRIGERATION WITH SOLAR THERMAL HEAT
As the refrigerant passes from the condenser toward the evaporator, the heat pump’s internal monitoring components will recognise that more than enough of the liquid refrigerant is of sufficient quantity and quality to allow the evaporator to be effective. With this excess cooling capability, the compressors can then slow down or stage off.
The energy savings begin as the solar energy replaces some of the energy previously consumed by the compressor(s).
There are many preconceptions regularly encountered by technicians regarding the solar thermal cooling process.
Preconception No 1: Solar thermal adds heat to the refrigerant, which, in turn, the condenser is required to remove
The fact is within the solar thermal cooling process there is no actual ‘additional’ heat added. In the majority of cases, the logic controls are designed to recognise and control the thermal energy in today’s modern systems. This is accomplished via a thermistor sensor, rather than through the previous method of using a pressure transducer, thereafter the logic control of such required to calculate the temperatures.
The thermistor linked control logic then modulates the required compressor’s speed accordingly and, as such, the process allows the solar panels to replace an element of the heat that would normally be generated by the compressor/s.
Solar thermal, when correctly integrated into the cooling process of a modulating system, along with true thermodynamic logic controls, will operate efficiently with most refrigerants.
The added benefit derived from the renewable, readily available solar energy may vary in comparison to the above illustrations, dependent on the refrigerant in use, along with the normally anticipated variables within any cooling system.
The condenser transfers heat from the refrigerant to the ambient air. The rate of heat transfer in a condenser is a function of its design properties, mass flow of the refrigerant, pressure, condensation temperature and the temperature and saturation of the refrigerant.
The solar thermal supported system retains the above process. The primary difference now being that on a system with the ability to modulate, the method increases the refrigerants temperature following the discharge from the compressor, while maintaining the pressure generated via the compressor. This therefore improves the Delta-T at the condenser point with a lower energy consumption at the compressor.
The alternative to this would be a compressor working harder to raise the pressure, subsequently raising the condensation temperature, along with an increased condenser fan speed.
Preconception No2: Heat = pressure
Some consider heat to be essentially an unwanted by product of the pressurisation process. This is also factually incorrect. The reality is that pressure is the unwanted, yet necessary precondition. Without heat, the cooling effect cannot be achieved. Pressure and heat are collectively vital sources in the refrigerant process, but it is also important to comprehend that in the modern-day modulation system, the thermodynamic method is vital for efficiency improvement. Therefore, these two factors rarely align for prolonged periods.
To further emphasise this point, the vast majority of today’s VRF/VRV/MDV systems are manufactured without a single pressure transducer linked to the operational logic controls. These are now predominantly thermistor sensors.
Preconception No3: Solar thermal HVAC works, but only works in high ambient temperatures
Although it is generally accepted by those who have an understanding of variable refrigerant technology that a solar thermal assisted system would be beneficial in high ambient temperatures due to the Delta-T benefits, a common misconception is that this would not be the case in the considerably more temperate or ‘normal’ ambient environments.
Consider the fact that when the sun is available, the thermal collector continues to provide thermal energy to the refrigerant. The variable system’s logic control recognises this fact via thermistor sensors, as if this added heat is provided by the compressor.
For example, to achieve the required Delta-T (liquid production), the system’s logic control measures data supplied via the thermistor sensors located at the condenser. Let us postulate that this specifies an increase in compressor demand, which on this occasion equates to a discharge temperature of, say, 65°c (149°F), along with the equivalent mass flow via the compressor. However, the temperature generated from the compressor and the solar array combined is, say, 70°c (158°F), maintaining the mass flow. The condenser logic sensors reasonably assume that the discharge temperature and subsequent mass flow is born only from the compressor output.
The reality is that the actual temperature discharged from the compressor may only need to be 40/45°c (104/113°F) with the relevant expected mass flow, due to the solar thermal supplementation. As such, the logic control may conceivably communicate to the compressor to slow down or maintain its position dependent on the available solar input, while maintaining mass flow of the refrigerant from the compressor.
The temperature generated via the compressor is determined by the logic to have increased, allowing the compressor to reduce its workload while actually providing a Delta-T in line with what would normally be achieved with the compressor working at a higher energy consumption rate. Ultimately, this results in an improved liquid refrigerant mass flow through the metering device, with an observed, measured and recorded reduction in energy consumption. All this results in reduced flash gas, or, as in the majority of cases, zero flash gas.
Of course, pressure remains an important and always required component in the liquefication process. In today’s modern thermodynamically determined systems, however, pressure is monitored primarily to ensure system protection, but rarely measured in relation to impacting the logic of the control’s decision-making procedure.
The solar assisted cooling system produces efficiency gains by allowing the compressor to slow down to stages as low as its lowest possible design point, due to the utilisation of the same method.
Preconception No4: The solar thermal system cannot produce efficiencies on a cooling system during the night
The sheer nature and design of the technology dictates that the solar thermal collector would generate zero additional heat energy following the discharge from the compressor during the hours of solar blackout. Consequently, it would be reasonable to surmise that there are therefore zero additional efficiencies gained during this period.
On the contrary, the efficiencies achieved are now achieved from the opposite effect, with the solar panels now acting as an oversized condenser, dissipating an element of the refrigerant’s heat prior to the condenser. The solar panels essentially reverse their role, again resulting in an improved liquid refrigerant mass flow through the metering device, delivering an observed, measured and recorded reduction in energy consumption.
Even 0vernight, efficiencies over 25% were found to occur. When the sun is no longer on the solar panel, it cannot increase the energy in the refrigerant. However, it doesn’t need because the outside air temperature also drops significantly at night. Therefore, the refrigerant does not need to be as hot to reject heat into the atmosphere. An adequate Delta T between the refrigerant and the outside air still exists. Due to the lower overnight outside temperature, the solar panel acts to extend the heat rejection surface area of the condenser.
The case studies below illustrate the benefits of solar thermal heating for HVAC.
CASE STUDY NO 1: SOLAR THERMAL COOLING RETROFIT FOR AN HVAC SYSTEM AT A LEISURE CENTRE IN ABINGDON
White Horse Leisure and Tennis Centre (WHL) is a state of the art leisure centre operated by Greenwich Leisure (GLL), located in Abingdon, Oxfordshire. The centre has a variety of facilities including gym, fitness class studio, group cycling studio, swimming pool, badminton & squash courts, sports hall, sauna & steam room & numerous tennis courts.
During the pre-installation energy audit phase, the energy usage of the VRF systems were assessed to prepare the financial feasibility study required for GLL to invest. The consumption of the 3 systems were measured over a 7-week period and the collective daily consumption profile is shown below.
Based on the input consumption and load profiles a conservative net energy saving of 21,002 kWh per annum was predicted for the solar cooling system. If achieved, this would result in a pay-back period in 4.4 years.
Lessons & Results
Following completion of the project the air conditioning systems were monitored further & below can be seen the first 4 weeks of data.
Average daily 261.29 kWh Estimated annual consumption (extrapolated) 95,370 kWh Approx. annual running costs £ 10,490
Average daily 149.54 kWh Estimated annual consumption (extrapolated) 54,582 kWh Approx. annual running costs £ 6,004
Overall savings 42.76 % Cash savings £ 4,486 Consumption savings 40,788 kWh ROI: 3.5 years
CASE STUDY NO 2: CHINGFORD FRUITS, FRUIT PACKAGING FACILITY – DARTFORD KENT
Colin Ormerod, Central Services Manager at Chingford Fruits said “we have been on the lookout for credible efficiency solutions for our refrigeration and air conditioning plant for some time. We came across the Solar Cooling Option and it really captured our interest.
Although the data logger shows a reduction in energy consumption of c. 92,500kWh over the evaluation period. Colin Ormerod also confirmed that production in the plant was actually much higher than the previous year in comparison, and therefore the true saving is more likely well in excess of 100,000kWh.
Usage data is monitored and stored by the client’s in-house t-mac technologies system, which measures all water, gas and electricity usage throughout the facility on an individual plant basis. The installation of the Solar Thermal system was commissioned on February 27th 2016, at this point direct year-on-year comparison began. The data below covers the period February 28th 2016 to May 10th 2016
CASE STUDY NO 3: TEXACO PETROL STATION
Another case study was completed at a petrol station in Burnley, where extensive pre-and post-retrofit monitoring was done on the systems serving the refrigerated display cases. Throughout the study, it was determined that the refrigeration system’s average daily savings in electricity consumption were running at 41.5% compared to levels before the installation of the solar thermal technologies. (See bar chart below.)
There are many more Case studies and data covering a number of well-known and recognisable end users, including an independent evaluation conducted by Toyota, along with many individual or multiple corporations, have been compiled. These are available on request.
Solar Cooling is a technology that is well proven, tried and tested and has been working successfully on a variety of systems throughout the UK, however it’s not suitable where equipment is oversized and running at part load or where there is insufficient space for the solar collectors. An assessment must be made as to viability and suitability
Large refrigeration systems as found in Data Centres, Supermarkets, Refrigerated distribution centres, manufacturing plants and Cold Stores are particularly well suited to the technology along with suitable Commercial offices. Incidentally the solar collectors do not have to be angled nor facing directly at the sun.
EnviroLogik would welcome customers who would like to visit to an existing installation and evaluate the savings and will be more than welcome to install our proprietary IOT Energy Consumption system to assess the suitability of a site.
This article was written by Chris Gunn of EnviroLogik.