Trouble with Loop in R - there must be a better way? - r

Question

I am new to R, and trying to get a handle on things for a school project.
The idea is to model a simple and hypothetical electricity generation/storage system which uses only solar panels and battery storage to meet energy demand. The aim is, given a predetermined storage capacity, to select the least amount of solar paneling that ensures that demand will be satisfied on every day of the year. I have a full-year of daily data - solar insolation figures that determine how productive panels will be, day-time electricity demand, and night-time electricity demand. Surplus generation during the day is stored in batteries, up to the predetermined limit, and then discharged at night to meet demand.
I have written a simple R program, f(x), where x is the amount of solar paneling that is installed. Like the battery-storage parameter, it is invariant over the entire year.
After creating new variables for the total power output per day and total excess power produced per day and adding these as columns 4 and 5 to the original data frame, the program creates two new vectors "batterystartvector" and "batterymidvector," which respectively indicate the battery level at the start of each day and at the midpoint, between day and night.
The program loops over each day (row) in the data frame, and:
(1) Credits the excess power that is produced (column 5) to the storage system up to the predetermined limit (7500 Megawatt hours in my example) - which is then stored in "batterymidvector."
(4) Subtracts the night demand (column 3) from the total just registered in "batterymidvector" to determine how much energy there will be in storage at the start of the next day, and stores this figure in "batterystartvector."
(5) Repeats for all 365 days.
Ultimately, my aim is to use an optimization package, such as DEoptimr, to determine the lowest value for x that ensures that demand is satisfied on all days - that is that no values in either "batterymidvector" or "batterystartvector" are ever negative.
Since every entry in the two battery vectors is dependent on prior entries, I cannot figure out how to write a program that does not use a 'for' loop. But surely there must be a simpler and less clunky way.
Here is the code:
library(DEoptimR)
setwd("C:/Users/User/Desktop/Thesis Stuffs/R Programs")
data <- read.csv("optdata1.csv", header=TRUE)
#x is pv installed and y is pumped-storage capacity
#default is that system starts with complete pumped reservoir
f <- function(x) {
data$output <<- (data$insolation*x)/1000
data$daybalance <<- data$output - data$day
batterystartvector <<- vector(mode="numeric",length="365")
batterystartvector[1] <<- c(7500)
batterymidvector <<- vector(mode="numeric", length="366")
for(i in 1:nrow(data)) {
#charging up
batterymidvector[i] <<- min(batterystartvector[i] + data[i,5], 7500)
#depleting
batterystartvector[i+1] <<- (batterymidvector[i] - data[i,3])
}
}