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Devap air conditioning
Desiccant-Enhanced Evaporative (DEVAP) Air Conditioner
"Summer is coming" -Electric Companies
1. What is the function of using a desiccant?
2. How does it help in air conditioning and reducing electrical costs?
Air conditioning (AC) may seem like a highly situational and innocent appliance, but it may become one of the world's top concerns in the future. Currently, AC is 15% of America’s electricity consumption and can be as much as 70% of use during hot summer days. The US uses the most energy in August and July which directly translates into the most expensive energy as well. The need and use of "peaker" plants for this energy demand costs the energy provider and the end-user a significant amount of money. In a more global perspective, we can see that d
eveloping nations such as China, India, Vietnam and South American countries have a high need for air conditioning. In 2007, only 11 percent of households in Brazil and 2 percent in India had air-conditioning, compared with 87 percent in the United States. These countries are also significantly hotter than the US as well. As developing nations grow their economies and income per capita, more AC units will surely be purchased. In India, especially, cooling units are saved for and socially correlated with status.
These current and future demands put this industry in a challenging position. Winter heating demands can sort of be averted by bundling up and insulating buildings. Summer heating can be difficult to escape from and certainly more so in hotter climates. I bring up this point because conservation efforts and behavior changes will just not be sufficient to curb this energy demand. Telling developing countries to simply not use energy on AC will just fail to due to perceived inequality (again comparing our 87% AC units per household). There is a need for a new technology that will avert energy use in this industry for current users and the vast untapped future users. This new technology will need to be comparable in size, cost and materials to the current standard yet use far less energy. This is usually attempted by trying to optimize the co-efficient of performance (COP). However, we found this interesting new technology developed out of NREL that could solve these issues. We consider this new DEVAP unit by investigating its cost, energy use and overall ability to seize control of the AC market.
First, we'll consider the toy model below. These are two glasses of water connected through a diffusion tube at the bottom. Each glass of water is given a desiccant to form a salty solution. If there is enough desiccant in the solution, it will draw moisture from the air into the solution. The two glasses connected through diffusion will maintain equilibrium; each glass will strive to have the same water and salt content. Now to make things interesting, the left cup is exposed to an energy source such as sunlight. This energy will evaporate water out of the left cup. In an attempt to maintain equilibrium, the left cup will draw more water from the right cup. To maintain this, the right cup must now draw even more water from the room. This is the basic system of desiccant-enhanced AC. A desiccant salt solution is used to dehumidify air, which makes the air easier to cool, and then the excess moisture is removed with a heat source. This idea of dehumidifying and regenerating the desiccant is fundamental to this technology. Instead of spending extra electricity trying to cool humid air or condense the water vapor (which would release the heat of vaporization into the air you're trying to cool), the entire dehumidifying step is done chemically.
You can see a schematic of a prototype DEVAP unit below. The unit uses the same principles as the two glasses model above. There is an indoor half (Cooling phase) and an outdoor half (Regeneration phase). On the cooling phase, air enters the system by a fan inside the building. The air is then passed through a honeycomb media that also has cool liquid desiccant flowing through it. This dehumidifies the air and cools it at the same time. The treated air then exits the system into a room that requires cooling. The liquid desiccant is pumped through a heat exchanger to provide cooling. It then flows over the honeycomb media (as shown below) to absorb water vapors from the intake air. The honeycomb media allows for the air and desiccant solution to interact in a highly optimized surface area while not contaminating the air with excess desiccant. The solution then enters a tank at the bottom of the unit. Just like the model above, the solution is bridge to the regeneration phase by a diffusion port. To create equilibrium of concentration between the cooling phase and the regeneration phase the water diffuses to the regeneration side of the unit.
On the regeneration side, the desiccant solution is once again pumped through another heat exchanger. This time the heat exchanger is used to heat up the desiccant solution to water boiling temperatures to evaporate the water from the desiccant. The hot desiccant solution then flows over a honeycomb media in which air from the outside passes through. The air picks up the evaporated water vapors and excess heat and exhausts it to the outside environment.
The cooling part of the system is indicated by the green arrows in the schematic below. Unlike what is shown, instead of using gases as a medium to transport heat and a compressor, we used the heating and cooling of water. This creates less electrical use by using natural gas instead as a heat source. The natural gas heats up a water tank to boiling or near boiling temperatures. The hot water is then pushed into a heat exchanger that heats up the desiccant on the regeneration side. The hot water is then cooled significantly by the heating of the desiccant and is then moved to the cooling side of the unit. The water is once again passed through a heat exchanger to cool down the desiccant on the cooling side. This is what cools the indoor air. The now heated water is then passed back into the heating tank to repeat the cycle. Additional heat exchangers can be used at different locations for more effective cooling. Also the heat source does not have be natural gas. Thermal heat can be achieved by a solar thermal solution or use electricity in areas that cannot acquire natural gas.
The differences between a DEVAP and DX (standard AC unit) are minimal as the DEVAP system is designed to mimic the performance and space of the DX units. The weight of the commercial and residential DEVAP units are very similar to DX units and pose no extra load on support structures. The DEVAP system requires a regenerating unit for the desiccant which can be placed outside, next to the DEVAP unit or combined to a furnace. The flexibility in the placement for the regenerator may allow for renewable heat sources such as solar thermal or geothermal. The entire system may be packaged together for residential units anticipating that this may be easier for installation. The main difference between these two types of units is in the design of the air flow. The DEVAP system requires outside air and return air as intake. However, it outputs hot and humid air that should not be released near either intake. The exhaust air would require an air duct if the unit is not placed near the outside. Overall, retrofitting is not a difficult or expensive process for either the commercial or residential case.
The new DEVAP system will also require new training for installations, maintenance and repair. This will not necessarily increase the price for services compared to DX AC repairs, but requires different knowledge and skills to maintain, such as: knowing how to repair desiccant lines, or knowing that the DEVAP system has two air filters instead of one. The DEVAP unit still maintains reasonable operations and maintenance costs. Retrofits can be harder due to extra ducts required to pull indoor and outdoor air. Some part of the unit has to have access to the outside environment which can be hard to achieve when retrofitting houses with indoor units. Also, because the unit uses thermal energy instead electrical energy for cooling, some house will need a natural gas line leading to the unit.
The DEVAP unit also has different impacts on the environment compared to the standard AC units. The DEVAP unit is made with mostly plastics which can be made from recycled materials. The DEVAP unit also does not use harmful chemicals in order to operate. DX units use environmentally harmful chemicals in their coolants such as hydrochlorofluorocarbons and chlorofluorocarbons which are harmful greenhouse gases and disintegrate our ozone. The DEVAP unit uses water as a medium for thermal transportation and the desiccant is made up of concentrated salt water such as lithium chloride or calcium chloride. The unit also provides clean disinfected air into the cooled room creating a better atmosphere for people who have sensitive allergies. Because the DEVAP units can get up to 90% electrical savings, this reduces the electrical load on power plants. This puts less stress on power generation facilities, which in turn means there is less of a chance that they will need to use older less efficient generators. On the other hand, DEVAP exhausts hot humid air when in operation. This can have a negative effect if they are used on a massive scale. There is only speculation of this effect as of now, but this could potentially raise humidity outdoors.
DEVAP AC unit
DX AC unit (with dehumidifier)
Above is a table of the estimated costs for a DEVAP unit considering all parts and at various sizes. The DEVAP unit costs do not include the possibility for utility incentives and are just the base materials, manufacture and installation costs. The problem in comparing with the DX unit is that the DEVAP system is more expensive in both circumstances. Particularly troubling is that the ratio of costs between the units in the residential sector is 1.6:1 for DEVAP:DX. The commercial is only 1.3:1, making the cost difference less at larger units. Businesses may be able to consider the energy savings in such a device, but residential buyers may have trouble justifying the extra costs. We will need to further explore the energy savings and cost per energy saved to justify the investment in such devices.
Above is a modeled energy consumption during peak hours for residential units in different locations. There is significantly less electrical use for the DEVAP units freeing up the stressed summer power grid. Residential savings are significant in terms of energy already, but residents aren't charged per peak usage in the first place. In this situation, the utility companies largely benefit from such an efficient device. We believe that utility companies could incentivise such a purchase in the residential sector through real time pricing of electricity (unlikely change) or through monetary compensation. More of these devices purchased, the happier the utility companies will be.
The plot above is an excellent display of the energy costs and annual costs per system in a residential unit. Due to the more expensive investment cost, the loan payment for 30 years at 7% interest is quite larger. You can easily see though that the sum of electricity, gas and water costs necessary to run the DEVAP system is far less than the electricity usage in the DX system. This is where the DEVAP system is advantageous because the desiccant performs the entire dehumidifying step for the cost of regenerating it (with gas in this simulation). Electricity costs are up to 94% less in the DEVAP system, which again saves money and peak power energy use at the hottest time of the day. The cost of conserved energy for the DEVAP system is $0.05 compared to the $0.07
(for Phoenix data and electricity rate). This makes it a sound investment for purchasing a new system. The problem is that retrofitting residential houses to switch to this technology is more financially troublesome. For retrofitting, banks will only offer a home equity lone that has a 5 year payback time. This means that the yearly loan cost is much higher and puts the total annual cost for the
system at about $2,200/yr. This makes the system more expensive and requires more upfront cost. Luckily your loan will be paid back faster, but the cost due to the retrofitting loan makes the system less competitive during the 5 year payback.
We can also do a similar analysis for commercial units. Below is a table considering another simulation for Phoenix using a 30 year mortgage, 7% interest rate, geographic water and electricity costs, no extra peak electricity fees, and assuming no energy incentives or rewards.
DEVAP 10-ton unit
Yearly Electricity cost
Yearly natural gas cost
Yearly Water cost (at $5/1000 gal)
Net Yearly Operational Cost
We can see clearly the benefits of using a DEVAP system. The difference in loan cost is about $500 while the usage costs have a $-2,000 difference. Since the capital investment for a large unit is more comparable to the DX system in the first place, the DEVAP unit is already more competitive compared to the residential analysis. At the commercial scale, the usage costs are far more than the actual loan costs for standard AC. The commercial DEVAP unit may clearly be a commercial success in the future if companies consider a similar analysis. The lifetime of the DEVAP system is estimated to be 11-15 years which is similar to the DX units. Depending on the industry, businesses may also be charged based on peak electricity use. DEVAP units would certainly be charged less and be even more cost effective.
The DEVAP system provides a good solution to the AC necessity outlined above. These units provide clean, cold and dehumidified air for far less electricity and overall energy by utilizing the chemical capabilities of desiccants.The DEVAP system was designed to replace the standard AC units so there should be minimal difficulty in terms of installation or retrofitting capability. The overall energy savings have been simulated to be between 40-80% depending on geographical location and humidity level. The energy efficiency and environmentally respectful chemicals make this a good solution to curb the greenhouse effect. The DEVAP system costs more for both residential and commercial units, but added operational costs make it financially advantageous within a 30 year span. Since most potential buyers mainly consider capital cost, we think it may be difficult to convince industries to adopt this technology. We believe that energy incentives from energy companies or the government will be effective motivation. The savings in peak energy for both the electrical suppliers and AC consumers should make it mutually beneficial.
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